Police have confirmed that the victims of a collision Sunday on the Trans-Canada Highway are three members of a family from Conception Bay South and an 18-year-old woman from the Burin Peninsula.

The three occupants of one of the vehicles — a 41-year-old man, a 35-year-old woman and an 11-year-old boy — were all pronounced dead at the scene.

Police said they were in a Chevrolet Equinox heading east when their vehicle crossed over two lanes into the path of a westbound Honda Pilot.

Traffic was backed up for hours in both directions. (Hope Green)

"We don't know if it was caused by a medical condition, by driver inattention, by aggressive driving, if it was something to do with driver distraction," said RCMP Staff Sgt. Boyd Merrill.

The 18-year-old woman who was driving the Honda, also died at the scene. Her passenger, a 16-year old male, was injured in the crash and brought to hospital in St. John's.

Merrill said the two SUVs collided head on, around 5 p.m. about four kilometres west of the exit to Bellevue.

. <a href="https://twitter.com/RcmpHolyrood">@RcmpHolyrood</a> says eastbound Equinox SUV with family of 3 pulled into path of oncoming Honda Pilot. Not known why. <a href="https://twitter.com/hashtag/cbcnl?src=hash">#cbcnl</a> —@TRobertst

Debris at collision site evidence is the violent impact that left 4 dead. <a href="https://twitter.com/hashtag/cbcnl?src=hash">#cbcnl</a> <a href="https://t.co/eHCnqKINEc">pic.twitter.com/eHCnqKINEc</a> —@TRobertst

He said the province's chief medical examiner is conducting a post-mortem examination and the vehicles will have to be checked for mechanical failure.

Other drivers reported wet road conditions as a possible factor.

Traffic was backed up in both directions for hours as crews worked to clear the scene.

The road was open again to traffic by 11 p.m. but police are still investigating and talking to witnesses. Cleanup resumed Monday morning to clear the area of any remaining debris.