Police and firefighters in Oakland, California are using surveillance tools like drones and DNA preservation, usually reserved for criminal investigations, to ascertain damage and identify victims from a deadly fire that broke out Friday night.

The blaze engulfed a local warehouse, dubbed “Ghost Ship,” which had been unofficially been converted to a music venue. The structure fire is believed to be among the worst in the country in recent years.

At a 3pm press conference on Sunday, Sgt. Ray Kelly of the Alameda County Sheriff’s Office said there had been 33 official deaths recorded, and he estimated that “35 to 40 percent” of the building has been searched, and was ongoing.