Police arrested a man “possibly in crisis” Wednesday afternoon after he allegedly stabbed someone who lives in his Rainier Valley apartment building.

Officers responded to the building in the 3500 block of Rainier Avenue South just after 4 p.m. for reports of a stabbing, according to the Seattle Police Department. When they arrived, they found a 55-year-old man with a knife in his back, police said.

Seattle Fire Department medics transported him to Harborview Medical Center in serious condition, a fire spokesperson said.

The suspect, a 29-year-old man who police say was an acquaintance of the victim, was taken into custody. Seattle police spokesperson Valerie Carson said it’s still unclear what led to the stabbing.

No further information was immediately available.