Police are investigating a stabbing aboard a King County Metro bus that left a man with serious injuries in Seattle’s Lower Queen Anne neighborhood Monday afternoon.

Officers and fire personnel responded to Queen Anne Avenue North and Mercer Street around 4 p.m., Seattle fire spokesman David Cuerpo said. The stabbing was prompted by an argument that broke out between people who knew each other, said Seattle police spokesman Mark Jamieson.

The victim, who’s approximately 40 years old, was taken to Harborview Medical Center in serious condition, Cuerpo said. Jamieson added later Monday night that the man’s injuries weren’t life-threatening.

About 20 people were on the RapidRide D line bus, which was headed toward downtown Seattle, at the time of the stabbing, Metro spokesman Jeff Switzer said.

One person was detained, Jamieson said.

No other information was immediately available.