A 24-year-old man allegedly stabbed another man to death Tuesday morning outside a McLean facility used for a program that provides day services for adults with mental illness, police said.

Mark D. Shelor, of Fairfax, was charged with murder in the death of 57-year-old Grady Alexander Vickers, of Falls Church, police said. Both were clients of the program, PRS, Inc., police said.

Officers responded to the scene in the 1700 block of Old Meadow Road shortly before 9:30 a.m. for a report of a stabbing, police said. The officers were led to the victim by staff at PRS, police said.

Vickers was transported to a local hospital, where he was pronounced dead a short time later, police said. The suspect remained on the scene and was taken into custody by officers, police said.

An initial investigation determined the two men had encountered each other on a staircase leading to an exterior door of the PRS facility and Shelor allegedly stabbed Vickers in the upper body, police said. The stabbing allegedly occurred following a conversation.

It remains unclear what sparked the attack and whether the suspect and victim were arriving at or leaving the facility, police said.

The Fairfax/Falls Church Community Services Board has a $1.3 million contract with PRS to provide services at the day center, said Belinda Buescher, the board’s communications director. The program has been a county contractor for over 35 years, Buescher said.

“We are not aware of police having been called to the Tysons facility before,” Buescher said.

The Web site for PRS, which was founded in 1963, says it “exists so that individuals with mental illness, emotional and/or behavioral disorders achieve personal wellness, recovery and community integration.”

The Web site says PRS clients are persons with severe mental illness, including schizophrenia, bi-polar disorder and major depression. PRS officials released a short statement:

“PRS extends its thoughts and prayers to the family and friends of the victim.  We are cooperating with the police investigation.  It would be inappropriate to comment further at this time.”

This post has been updated.