A man drove his car into a group of pedestrians at a US university campus and began stabbing people with a butcher knife before he was shot to death by a police officer.
Police are investigating whether the incident at Ohio State University - in which nine people were injured, one critically - was a terrorist attack.
The attacker was identified as Abdul Razak Ali Artan, a Somali-born legal permanent resident of the US, according to an official.
The FBI and other agencies have joined the investigation.
Details emerged after a morning of confusion and conflicting reports that began with the university issuing a series of tweets warning that there was an "active shooter" on campus near the engineering building and that students should "run, hide, fight".
The warning was apparently prompted by what turned out to be police gunfire.
Numerous police vehicles and ambulances converged on the 60,000-student campus, and authorities blocked off roads.
Students barricaded themselves inside offices and classrooms, piling chairs and desks in front of doors.
Ohio State Police Chief Craig Stone said that the assailant deliberately drove over a kerb outside a classroom building and then got out and began attacking people with the knife.
An officer who happened to be nearby because of a gas leak arrived on the scene and shot the driver in less than a minute, Stone said.
Angshuman Kapil, a graduate student, was outside the building when the car barrelled onto the pavement.
"It just hit everybody who was in front," he said. "After that everybody was shouting 'Run! Run! Run!'"
The "shelter-in-place" warning at Ohio State was lifted and the campus declared secure after about an hour and a half, after police concluded there was no second attacker, as rumoured.
At least two people were being treated for stab wounds, four were injured by the car and two others were being treated for cuts, university officials said.
The attack came as students were returning to classes following the Thanksgiving holiday break and Ohio State's football victory over rival Michigan that brought more than 100,000 fans to campus on Saturday.
Rachel LeMaster, who works in the engineering college, said a fire alarm sounded on campus.
"There were several moments of chaos," she said. "We barricaded ourselves like we're supposed to since it was right outside our door and just hunkered down."
LeMaster said she and others were eventually led outside the building and she saw a body on the ground.
The initial tweet from the university's emergency management department went out at around 10am and said: "Buckeye Alert: Active Shooter on campus. Run Hide Fight. Watts Hall. 19th and College."
Ohio State president Michael Drake said the active-shooter warning was issued after shots were heard on campus.
"Run, hide, fight" is standard protocol for active shooter situations. It means: Run, evacuate if possible; hide, get silently out of view; or fight, as a last resort, take action to disrupt or incapacitate the shooter if your life is in imminent danger.