Provincial police are appealing for witnesses to a shooting that left three people hurt on Highway 401 in Pickering Friday morning.

A man was seriously injured in the incident and remains in hospital. Another man and a woman were treated for minor injuries and have since been released, says OPP Sgt. Kerry Schmidt.

Schmidt says the shooting happened around 3:20 a.m., but investigations are unsure exactly where on the highway it occurred.

The car the three victims were travelling in was found on the shoulder of the westbound collector lanes near the exit for Whites Road. The drivers-side front window was shattered. The vehicle will be towed to the OPP's forensic lab for further analysis.

Police are so far unable to say if it was a random act or a targeted shooting, Schmidt adds.

"We are working on getting more information and talking to witnesses," he told reporters at a late-morning news conference near the scene.

The OPP criminal investigations branch is leading the probe.

Schmidt says the highway will remain closed for several more hours as the forensic identification unit works to collect evidence.

"There are still a lot of questions that need to be answered about how this came to take place on the highway early this morning and what those circumstances are."

Anyone with potentially valuable information or dashcam footage is encouraged to contact police or leave an anonymous tip with Crime Stoppers.