IRVINE, Calif.—A 24-year-old man was arrested Monday, July 8, on suspicion of killing an 18-year-old man at a sober-living facility in Irvine.

Police were called just after 6 a.m. Monday to 2 Tyler Place, where the victim was found seriously injured, according to Kyle Oldoerp of the Irvine Police Department. The victim, whose name was not immediately released, was taken to a hospital where he was pronounced dead.

The suspect was arrested in Barstow with help from the California Highway Patrol, Mr. Oldoerp said.

The suspect and victim both lived at the sober-living facility and were possibly roommates. A motive for the crime was not immediately known.

The exact cause of death was still under investigation.

The killing was the second in Irvine—annually hailed as one of the safest cities in America—to occur in three months. On May 16, a UPS driver was fatally shot, allegedly by a co-worker who was a longtime friend of the victim.