SEATTLE — When Kaleb J. Cole landed at Chicago O’Hare International Airport after a trip to Europe last year, federal officials were waiting at the gate for a chance to question him. In his luggage was the trefoil flag of a neo-Nazi hate group. On his phone, a photo of two people posing at the site of the Auschwitz death camp.

The officials did not charge Mr. Cole with any crimes that day, or in the months to come, despite information that he was a leader of the Atomwaffen Division, one of the most violent extremist groups in the country. But last month, according to records provided by a prosecutor’s office Thursday, the authorities in Seattle moved to seize a cache of weapons from Mr. Cole, using a state law intended to prevent gun violence.

“This was an individual who had access to firearms and was preparing for a race war,” Kimberly Wyatt, a prosecutor in King County, Wash., said in an interview on Thursday.

The move was part of a larger effort by investigators around the country, including the F.B.I., to crack down on members of Atomwaffen, as officials seek to counter the rising threat from hate groups. The Atomwaffen Division has been linked to a series of killings.