Three students of Carleton University in Ottawa were killed in an overnight crash with a city bus and police say alcohol may have been a factor.

Mark MacDonald --the 20-year-old driver of the car -- died at the scene, as did 19-year-old passengers, Brianne Deschamps and Vanessa Crawford. Another 19-year-old woman has been released from hospital.

They were in a Toyota SUV when it collided with a city bus around 1:50 a.m. at one of Ottawa's busiest intersections.

One other victim, Ben Gardiner, is in hospital with critical injuries. This is the second time Gardiner has been a passenger in a fatal car crash. The first happened when Gardiner was a student at Arnprior District High school, west of Ottawa. A friend of his died in that crash on Halloween of 2003.

The OC Transpo bus was travelling northbound on Riverside Drive and the SUV was travelling westbound on Heron Road when the crash occurred.

The driver of the bus, who was treated at the scene for minor injuries, told investigators he had a green light as he headed through this intersection.

After colliding with the SUV the bus careened into the traffic light. Because the traffic light was destroyed, investigators will be able to determine who had the right of way at the time of impact.

"We believe alcohol was a factor in this collision but it is still yet to be conclusively determined by the investigators," Ottawa Police Service Const. Alain Boucher told CTV Newsnet on Wednesday.

Boucher said the three killed were pronounced dead at the scene.

In a statement posted on Carleton University's website, President Samy Mahmoud said he has asked that the flags on campus be lowered to half-mast.

"I was saddened to learn this morning of the tragic vehicle accident that took the lives of three members of our university community," said Mahmoud.

"This news is devastating for all of us. On behalf of Carleton University's staff, faculty and students, I would like to extend my sincere condolences to the families of the three victims and hope and pray for the recovery of the other two people involved."

Boucher said he wasn't aware of any information that road conditions may have been a factor in the crash.

The intersection was closed during the morning commute but has now been reopened. The police investigation is ongoing.