Metro Transit Police are on heightened alert after suicide bombers killed at least 37 people in the Moscow subway system Monday morning, a Metro official said.

Metro Transit Police Acting Chief Jeri Lee said Metro Transit Police K-9 Explosive Ordnance Detection Teams will randomly sweep Metro stations and rail yards throughout the day, according to a Metro press release. Locations of the station sweeps will not be announced in advance.

“When we opened the Metro system this morning, we did so with heightened security,” Lee said in the release. “We remain an open system and we do what we can to be as secure as possible.”