PORTLAND, Ore. - A man was stabbed on a crowded TriMet bus during rush hour Friday after getting into an argument with another rider.

The 40-year-old was stabbed in the stomach with an unknown weapon and was taken to an area hospital. He was expected to survive, police said.

According to police, the man was having a loud, ongoing argument with a younger man during the bus ride. It was not clear what the argument was about.

When the bus stopped near the intersection of Southeast 44th and Burnside about 5:15 p.m., the younger man left the bus. Riders then noticed the 40-year-old slumped over in a seat, and then they discovered he had been stabbed.

No one apparently saw the stabbing happen. Police were called to the scene, but the younger man had already left the area, police said.

Officers searched the area in vain for the man.

Police said he was about 25 years old and was wearing a white do-rag. They asked anyone who was riding the bus and recognized the man to call police.

Officers also believe surveillance video on the bus might have captured the man's image.