The last hours of Baltimore’s Key Bridge: Dark. Quiet. Then chaos and tragedy.

Hundreds of times a year, gargantuan vessels like this one, laden with containers of freight, had eased beneath the Key, bound for open ocean and ports around the globe. But as the Dali reached eight knots and drew closer to the bridge, what should have been a routine passage suddenly wasn’t.

First, the giant ship’s lights blinked out. About a minute later, electricity returned, then the lights went off again.

The ensuing catastrophe will long reverberate — in coming months of economic disruption, vast rebuilding costs and incessant commuting headaches — but most profoundly in the lasting anguish of families, in their grief for lost loved ones.

Unable to maneuver with its power gone, the Dali plowed forward, borne by its momentum, and slammed into one of the bridge’s support piers, collapsing the towering metal structure into the river. Down with it into the frigid Patapsco went eight workers. Many of the workers were immigrants from Latin America, six of whom are confirmed or presumed to be dead.

If not for two police officers on either end of the span who each halted 25 to 30 vehicles after a radioed distress call from the unnavigable ship, probably more would have perished.

What brought these people all to one place in Baltimore Harbor — the Dali’s crew, the bridge laborers and the officers now hailed as heroes — were weeks and days and hours of routine. They were out there after midnight doing their jobs: fixing pavement, safeguarding the public and conveying goods to the world by the thousands of tons; it was just another shift on the time clock before the lights on that freighter turned dark.

This account of the Key Bridge calamity, and of the preceding hours and lives of those caught up in it, is based on numerous interviews, statements by police and government officials, publicly available records, video tracking images and other sources.

The Dali, 984 feet long and 157 feet wide, had docked at the Port of Baltimore three days earlier.

Its 21 crew members, 20 from India and one from Sri Lanka, had traversed an enormous distance to get there. They departed Busan, South Korea, on Feb. 21, passed through the Panama Canal, then journeyed to New York and Norfolk before arriving in Baltimore. Next would be another month-long voyage, to Sri Lanka, with a new load of cargo. In the meantime, the shipmates waited in a city far from their homes.

Baltimore-area members of a Catholic group called the Apostleship of the Sea of the United States reached out to the crew to ask how they could help make the stay more comfortable. The organization’s mission, its website says, is to assist and minister to “mariners, fishermen, their families, and all who work or travel on the waterways of the world.”

The Dali mariners’ urgent request: Could someone take them shopping?

That was how Shawn Day, an Apostleship volunteer, found himself acting as a chauffeur Sunday afternoon for the Dali’s captain, the first officer, the first engineer and a ship’s electrician. They stopped at a Walmart and a Target, stocking up on personal supplies. Then it was on to a Best Buy, where several of the men bought laptops to give to their families.

As they drove, Day and the seafarers chatted about the differences between American and Indian foods. Day, who lives on Maryland’s Eastern Shore, told the captain about a goose his neighbor had killed and served to him for dinner. When they arrived back at the Dali, the seamen invited him aboard for a meal of vegetables and rice. But Day, exhausted by then, gazed up at the steep ladder leading to the ship’s deck and said he’d take a rain check.

On Monday, hours before the Dali was scheduled to shove off in the night, the captain texted the Apostleship, asking for another shopping trip, saying he needed a few more things. Andrew Middleton, the group’s local director, drove the skipper and a crew member back to Walmart, and along the way he listened as the captain described the Dali’s next crossing.

He had been plying the oceans for 30 years, this captain, and had a 15-year-old daughter waiting for him at home. The voyage from Baltimore to the Sri Lankan port city of Colombo would last 27 days, more time-consuming than usual, he told Middleton. Instead of crossing the Mediterranean and transiting the Suez Canal to the Red Sea, where Yemen’s Houthi rebels had been attacking ships, the Dali would be forced to follow a longer route, around the southern tip of Africa.

“They were going out of their way to ensure their own safety,” said Middleton, who later prayed that the crew would arrive in good health.

He knew the hardships facing such crews. Modern-day pirates. Loneliness and strenuous labor. Days of boredom and moments of danger.

“Have a safe voyage,” Middleton said to the men as they parted company, and he implored the Almighty, “God bless these gentlemen.”

On the bridge

About 150 longshoremen were at work on the dock beside the Dali shortly before the ship’s departure, using heavy machinery to hoist the last freight containers aboard around midnight as the crew prepared to get underway, according to union officials.

When they were finished, many of the stevedores drove home along Interstate 695, crossing the Patapsco River on the Key Bridge and skirting past another crew of workingmen — this one laboring on the road with shovels in the glare of floodlights.

Below them, tugboats nudged and pulled the Dali until it cleared the dock and was out on the river. Then the ship was on its own. Local harbor pilots said the rest should have been relatively easy: It’s a straight course along the Patapsco to the Chesapeake Bay and the Atlantic, with ample room for passage under a gently arching bridge that had spanned the waterway for 47 years.

At 1:24 a.m., the Dali’s voyage data recorder, a ship’s equivalent of a jetliner’s black box, stopped logging sensor data, possibly because of the power failure, federal investigators said. They said that the recorder’s audio component, using a redundant source of electricity, kept operating, and that it captured a cacophony of alarms blaring on the vessel’s command bridge.

Such power outages are called “full blackouts.” Sea captains who have experienced them recall being enveloped in eerie silence as the ship’s illuminated instruments and screens abruptly go dark and its humming ventilation system shuts down. In video recordings made from a distance, the Dali seems to vanish on the river, becoming one with the night.

Investigators are now exploring what happened, examining everything from possible “dirty fuel” that might have caused the power outage to questions about the bridge design and its ability to withstand such a blow.

Chartered by a Danish shipping giant, flagged in Singapore and operated by a Singaporean company, the Dali can carry nearly 10,000 containers — though it had only about 4,700 at the time — and more than 100,000 metric tons. Now all of it, the mammoth freighter and its staggering weight of cargo, was adrift and unstoppable as it bore down on the bridge, headed for one of the immense concrete support piers rising from the water and holding up the span.

And a call went out from the ship: Mayday.

High above on the bridge span, the laborers were hours into their shift.

On a typical night, they would meet around 9 p.m., at a fenced-in equipment lot owned by their employer, Brawner Builders, about five miles north of the harbor, where the company keeps its heavy vehicles, its cranes and dump trucks. After loading gear for the evening’s work, they’d get on the road in Brawner’s red pickup trucks, bound for the job site.

They were immigrants — men from Guatemala and El Salvador, Honduras and Mexico, trying to make their way in America, land of the paycheck.

“The kind of work he did is what people born in the U.S. won’t do,” said Hector Guardado, a nephew of one of the half-dozen laborers who would not survive the night.

Up on the bridge, rule No. 1 was safety, according to Brawner Builders. Clad in red hard hats and yellow vests, the men would close a traffic lane with orange cones and work behind the protection of an “attenuator truck,” a sturdy vehicle rigged with a shield to absorb the impact of an oncoming car, always a looming threat.

Motorists would whiz past, and sometimes the bridge, by design, would ever so slightly sway.

Usually by 10:30 p.m. at the latest, other Brawner employees said, the crew would be hard at work on the pavement, trimming potholes with bulky power saws, until the edges were straight and sharply cornered. While the saws screeched, men would mix cement in five-gallon buckets to be poured into the neatly squared divots in the road. Then, in the waterfront wind and chill, would come the tedium of waiting for the concrete to set.

Some brought home-cooked food. Others would be dispatched to buy snacks at a gas station or a Dunkin’ doughnut shop just off the bridge.

They were husbands, parents and sons, many in their 20s and 30s: Miguel Luna, a Salvadoran father of three, an avid soccer fan, had worked for Brawner for more than 10 years; Alejandro Hernandez Fuentes was a Mexican immigrant; Dorlian Ronial Castillo Cabrera was Guatemalan; Maynor Suazo Sandoval had come from Honduras. They were here partly for the sake of relatives they had left behind. The money they earned on the bridge would go a long way back home.

Now those four and two others are dead or unaccounted for.

Suazo Sandoval was Hector Guardado’s uncle. Even on the coldest nights, Suazo Sandoval would tell his nephew, the work helped to support not only his 5-year-old daughter and teenage son in Maryland but also loved ones in Honduras, a country he greatly missed. When there were baptisms or weddings in the family in Central America, Guardado said, Suazo Sandoval would dial in remotely, often crying as he listened.

He ordered cakes to be delivered for birthdays and medicines for the sick.

“Everyone else always came first,” said Guardado.

On the ship

On the ship, a desperate effort was underway to avoid a crash.

Foreign-flagged vessels such as the Dali are required to have local pilots on the command bridge as they enter and leave U.S. ports. In Tuesday’s pre-dawn darkness, two Maryland pilots were aboard the Dali as it gathered speed on the river.

The main pilot had more than a decade of experience and intimate knowledge of Baltimore’s ship channels, according to Clay Diamond, executive director of the American Pilots’ Association, who has been briefed on their actions. He said the other pilot was an apprentice who began training last month.

The pilots and crew faced compounding crises, Diamond said. The Dali had lost both engine and electrical power, crippling its controls and communications system. A backup diesel generator restored electricity, Diamond said, but not the ship’s engines. The pilot quickly reached a dispatcher at the pilots’ association, urgently warning that the Dali was out of control and that the Key Bridge needed to be cleared of people immediately.

“Have them shut down the bridge,” Diamond quoted the pilot as saying.

For the next two minutes, the pilot took the only available steps to slow the vessel and alter its course, federal investigators said. The pilot asked by radio for tugboats to help, ordered the Dali’s rudder turned hard and called for its anchor to be dropped. Meanwhile, the dispatcher alerted the Maryland Transportation Authority Police, and word of the emergency reached the officers on traffic-control duty at either end of the bridge.

The duty officer, 19-year veteran Cpl. Jeremy Herbert, was at a police station when the mayday call was relayed to him. He quickly contacted the two officers at the bridge, who were out in the cold for the overtime pay, slowing traffic to protect the construction crew.

“I need one of you guys on the south side,” Herbert suddenly ordered at 1:27 a.m. in a radio call that was recorded online. “One of you guys on the north side. Hold all traffic on the Key Bridge. There’s a ship approaching that just lost their steering. So until you get that under control, we got to stop all traffic.”

A “10-4” reply for Officer Garry Kirts came instantly: “I’m en route to the south side,” he said. “I’m holding traffic now. … I’ll have all outerly traffic stopped.” Kirts, who joined the force six years ago, and the other officer, Sgt. Paul Pastorek, a 13-year veteran, halted dozens of vehicles from driving onto the span, the president of their labor union said.

At 1:28 a.m., Herbert radioed the officers again: “Is there a crew working on the bridge right now?” The worry in his voice was mounting. “Just make sure no one is on the bridge right now. … There’s a crew up there, you might want to notify whoever the foreman is. See if we can get them off the bridge temporarily.”

Pastorek, on the opposite side of the bridge from Kirts, wanted to drive onto the span to alert the construction workers. But he radioed that before he could leave, he first needed a backup officer at his location to keep traffic stopped while he hurried onto the bridge.

At 1:29 a.m., waiting for the backup to arrive, he said on the radio: “Once you get here, I’ll go grab the workers on the Key Bridge.”

But it was too late. Only 10 seconds went by before Kirts was back on the radio, seeming to grope for words to describe what he had just witnessed.

He told the dispatcher to send help. “Start, start, whoever, everybody,” he said.

“The whole bridge just collapsed.”

Teo Armus, Sarah Cahlan, Erin Cox, Tim Craig, Scott Dance, Ian Duncan, Razzan Nakhlawi, Danny Nguyen, Imogen Piper, Jon Swaine and Rachel Weiner contributed to this report.