Florida - Florida shooting suspect Nikolas Cruz was a member of a white supremacist group and had taken part in military-style training exercises, the rights group Anti-Defamation League said on Thursday.

Jordan Jereb, believed to lead the shadowy Republic of Florida (ROF) group, told ADL that Cruz had been "brought up" by a member of the group and joined its training exercises near Tallahassee, Florida.

Armed with an AR-15 assault rifle, Cruz is charged with killing 17 students and adults at Marjory Stoneman Douglas High School in Parkland, Florida.

Republic of Florida describes itself on its website as a "white civil rights organisation fighting for white identitarian politics". It advocates creating a "white ethnostate" in Florida.

ADL said the group "borrows paramilitary concepts from the anti-government extremist militia movement".

Jereb told ADL that Republic of Florida had not ordered or supported Cruz's attack on his old high school.

In an interview with the Daily Beast, Jereb said Cruz "seemed like just a normal, disenfranchised, young white man".

"I don't know precisely what he believes," Jereb told the Daily Beast. "I know he knew full well he was joining a white separatist paramilitary proto-fascist organisation."

Jereb told the Daily Beast that Cruz was a member of its "cell" in Clearwater, Florida, about 300km northwest of Parkland, and had trained with them.

"He probably used that training to do what he did yesterday. Nobody I know told him to do that, he just freaked out," Jereb said.