Investigators say they believe the TTC vehicle rear-ended the truck. (CBC)

Toronto police now say the driver of the bus that collided with a truck on Tuesday, killing a passenger on the bus, was in possession of illegal drugs.

Police say the driver had a quantity of what is believed to be marijuana in his possession at the time of the crash — though they can't say if the drugs played any role in the collision.

Drugs were found in the driver's possession, although not on his person at the time, police said.

"Amongst his belongings, not specifically in his possession but amongst his belongings, there was some drugs found and they have been sent off for analysis," Det. Carl Andersen told CBC News.

In a news release the TTC said it had begun "disciplinary proceedings against a TTC bus operator immediately following a fatal collision on Aug. 30. The employee was relieved of duty following the crash this past Tuesday."

The commission said it had only learned late Tuesday evening that police had found the illegal drugs.

The TTC added, it found "no evidence of impairment on the part of the operator."

"The Toronto Police Service is leading the investigation into the crash and the allegation of drug possession. Police will determine what, if any, charges will be laid as a result of the crash and alleged drug possession," the release said.

The driver of the bus has not been identified.

No charges have been laid but police say they intend to consult with Crown prosecutors next week.

On Tuesday afternoon the bus collided with a flatbed truck on Lawrence Avenue East, at Railside Drive, near the Don Valley Parkway.

The cause of the accident is not clear. The flatbed Ford truck and the bus were both travelling westbound on Lawrence Avenue East, when the truck was rear-ended by the bus, "causing it to crash into the guardrail of the northbound Don Valley Parkway on ramp," said a news release from Toronto police.

Neither driver was seriously injured. Police said both drivers were co-operating with the investigation.

A 43-year-old woman, Jadranka Petrova, a passenger riding at the back of the bus was killed in the accident.

Thirteen other passengers suffered injuries in the accident.