While federal policymakers debate a controversial proposal that would separate asylum-seeking families after they reach the U.S. as a deterrent to future migrants, some immigrant rights advocacy groups say they're already seeing increases in parents separated from children along the southwest border.

Several nonprofits filed a complaint earlier this month with the U.S. Department of Homeland Security documenting at least 15 cases of asylum-seeking families split apart and sent to different detention facilities. Not all of the cases described in the complaint are indicative of a policy change, but circumstances in some of the cases suggest that the new proposal is already having an effect.

The complaint asks two oversight divisions of the department to investigate.

"The separation of parents from their children at the U.S.-Mexico border and within the United States, absent a justifiable child protection grounds, is so fundamentally unconscionable it defies countless international and domestic laws on child welfare, human rights and refugees," the complaint says.

An official with Customs and Border Protection declined this week to comment about the complaint.

"As a matter of policy, we do not comment on pending investigations," the official said. "However, lack of comment should not be construed as agreement or stipulation with any of the allegations."

Adults seeking asylum in the U.S. are generally held in immigration detention after they arrive at the border. The federal government has a limited number of facilities capable of housing women with children and even fewer for men with children, so families are often released more quickly than adults who are detained by themselves.

Beyond the trauma of being separated from a loved one, asylum seekers who are split from family members can also end up with different outcomes in their immigration cases because they are no longer linked in court. That means a child could win his case and stay in the U.S. while the parent loses and is deported, or vice versa.

Reports that federal authorities were considering family separation as an official policy first surfaced in early March. By the end of the month, after public backlash, then-Homeland Security Secretary John Kelly walked back the idea.

It resurfaced as a proposal just before Christmas, according to The Washington Post, when end-of-year statistics showed numbers of arriving families and unaccompanied minors returning to levels seen before President Donald Trump's inauguration.

In November of last year, 1,356 people traveling with family members either came to a San Diego port of entry without documents to enter the U.S. or crossed illegally and were caught by Border Patrol, according to data from Customs and Border Protection. That number dropped to 267 in March 2017.

At the time, officials celebrated the change as proof that Trump's immigration policies were working as intended.

By November, the number had climbed to 1,202.

About 70 percent of families arriving at San Diego's border in the last year went through a port of entry rather than sneaking in.

The complaint filed in mid-December suggests that though the policy to separate families has not yet been approved by the highest levels of the department, some families are already feeling its effects.

Four fathers described in the complaint were together in a temporary processing cell in San Diego in mid-November when immigration officials forced them to hand over their children, whose ages ranged from one to 12.

Immigration and Customs Enforcement, the agency involved in the incident, said that its officers could not verify the relationships between the men and the children. The agency said that some smuggling organizations have tried pairing adults with unrelated children to avoid being detained when they try to enter the U.S.

"The only thing I know for certain is that I was looking for help from this country, which is my right," said Eric Matute Castro, whose 3-year-old son was pulled from his arms. "I feel powerless. I came here to protect my son and to protect myself."

The men said the officials who took their children told them that the decision came from higher up.

The four fathers, three from El Salvador and one from Honduras, all said they fled gang violence in their home countries that included extortion and death threats.

Some of the parents listed in the complaint were separated from their children because they were charged with illegal entry or re-entry and placed in U.S. Marshals' custody while criminal proceedings are pending.

Advocacy organizations have pointed out that under a 1951 convention on refugees, countries are not supposed to penalize refugees or asylum seekers for illegal entry or being in the country without permission.

In other cases listed in the complaint, those who came to the U.S. as a family group – including mother, father and child – probably would have been separated in the past even without the Trump administration's suggested policy. In those instances, children stayed with the mother, and the father was detained separately, as were any other adult family members.

A Guatemalan woman's case suggests that this, too, may be changing to further separate parents and children.

She said in the complaint that she was separated from her two children and husband in September at the San Ysidro port of entry.

The children, ages five and 14, are currently in a shelter in New York, according to the complaint, while both parents are held separately at Otay Mesa Detention Center.

She has a phone number to call the shelter where her children are staying.

"When I do talk to my kids, they tell me they don't want to be there, they miss me, and they want to be with me," she said.