A device that shuts a door when it detects a weapon or other metal object was used this week in Fairfax County to stop would-be bank robbers, according to police.

They said the incident occurred about noon Wednesday after three men walked through the outer doors of the Union Bank on Springfield Boulevard in the Springfield area.

In a vestibule of the building, police said, the men put on masks. With Halloween far off, police said Friday that they think the men's purpose was robbery. If so, the group did not succeed.

Instead, police said, a metal detector inside the doorway was activated. Exactly what set it off was not described, but police said a second set of doors leading into the bank automatically locked.

The men still tried to get in, police said, but could not, so they left. Police said they are looking for the three.