NIAGARA FALLS, N.Y. (AP) — Authorities still don't know what caused 21 law enforcement officers to become ill after participating in a raid on a Niagara Falls apartment.

The FBI and Niagara County sheriff said in a statement Thursday that tests came back negative on materials collected by hazardous materials teams 12 days after the raid.

Thirty-nine members of federal and local law enforcement took part in the police activity on Dec. 14. In the following days, 21 people reported getting sick, including officers who didn't go inside.

They all have recovered.

Health officials continue to investigate.