The Rosewood Center, where the shooting occurred, dates to 1888 and once housed as many as 3,000 patients with developmental disabilities. Gov. Martin O'Malley ordered its closure in 2010.
Baltimore County police "use it both for recruit training and for specialized training for existing officers," spokeswoman Elise Armacost said in an email.
Armacost said the training often involves simulated, so-called "active shooter" exercises with paint-filled bullets. "No ammunition or live weapons are allowed," she said. "A safety officer is assigned to each exercise."
It was unclear what type of guidelines or agreements, if any, police need to follow to borrow the building from the state.
The state Department of Health and Mental Hygiene, which oversees the publicly owned facility, declined to provide any information unrelated to the investigation about the property, including what agencies are allowed to use it and under what type of agreements or contracts.
Baltimore Sun reporter Luke Broadwater contributed to this article.