Federal law enforcement officials have called in help from national investigators to probe a large fire that engulfed a vacant Minneapolis apartment complex on Dec. 3.

William McCrary, special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) St. Paul division, said Monday that those already investigating the blaze at the four-story apartment complex at 2312 Lyndale Av. S. "collectively decided" to call in the ATF's National Response Team because of "extensive fire damage, including a partial collapse, and the size and scope of investigation."

"Investigating fires in the winter can pose unique challenges, and the National Response Team resources can help with some of those additional winter-imposed hurdles," McCrary said in a statement. "The team's mission here is to collaboratively assist the Minneapolis Fire Department, the Minneapolis Police Department and the Minnesota Department of Public Safety State Fire Marshal Division, in the determination of the cause of the fire."

The ATF said the fire is estimated to have caused $1.8 million in damage. Firefighters spent hours extinguishing it after responding to reports of the fire early Saturday morning. The building is owned by C. David George, who previously had a condemned building in Loring Park damaged by fire in September.

Nobody was hurt in either fire, but officials said squatters inhabited the buildings. In the Lyndale Avenue fire, a person jumped from the second floor to escape.

George's Loring Park property had been condemned in 2021 for safety hazards and the city charged him $39,000 in special assessments, including costs of boarding up and reboarding the building. Neighbors have meanwhile complained to the city and neighborhood association about the property's deteriorating condition.

Saturday's fire at the Lyndale Avenue building prompted the ATF to call in its specialized national team of investigators back to Minnesota for the first time since 2020. That year, the team helped with a number of major cases — including more than 150 arson investigations across the Twin Cities connected to the civil unrest following George Floyd's murder.

"Investigating a fire of this size is time consuming and requires an all-hands-on-deck approach," Minnesota State Fire Marshal Jim Smith said in a statement. "The partnership of our federal and local agencies is critical and always appreciated, especially with a fire of this magnitude."

Star Tribune staff writer David Chanen contributed to this report.