Transportation Safety Board investigators are combing through the wreckage of a chartered cargo plane, trying to determine the cause of the crash that killed the two on-board pilots in Vancouver.

The twin-engine aircraft was en route from Victoria, B.C., to Vancouver International Airport when it went down on final approach just after 10 p.m. PT on Thursday night.

A spokesman for the Transportation Safety Board said the plane was about four kilometres from the runway when it exploded.

It was on the flight path but the wreckage trail is pointing off the heading to the runway," Bill Yearwood said Friday.

Yearwood said the Piper Navajo was descending and radar contact was lost at about 300 metres.

The plane had been transporting blood for Canadian Blood Services at the time of the crash, RCMP Sgt. Peter Thiessen confirmed Friday.

Reporting from the scene, CTV B.C.'s St. John Alexander said witnesses described a sudden, fiery crash.

"One said it was the sound the plane was making that made him look up," Alexander told CTV's Canada AM.

"The change in thrust, in the propeller speed -- he saw the plane with its nose down and within seconds it was on the ground. When he got to the scene, he tells us, he saw fire. He saw cars on fire, he saw a fence on fire but there was so much destruction he could not see a plane."

Liz Da Silva was travelling in a car when she saw the plane plunge into the ground.

"It was very quick and very loud," Da Silva told CTV News Channel during a phone interview from Vancouver on Friday morning.

"It was too low and it was going too fast and all of a sudden it just dipped and it crashed and it was a huge, huge explosion -- loud enough to hear through the car with the windows all up. And it was a big orange cloud and tons of black smoke started pouring into the sky."

Da Silva said emergency crews were "amazingly fast" in their dash to the scene, arriving about five minutes after the crash happened.

The plane crashed near the rear of the IKEA store in the Vancouver suburb of Richmond, B.C., about five kilometres from the airport.

The store was closed, and no one on the ground was injured, Alexander said.

Several cars that were parked outside a transmission shop were destroyed.

Darren Van Leeuwen, a witness who saw the plane go down, told The Canadian Press that flames reached about 50 metres into the air following the crash.

The plane was a Piper Navajo, which is capable of carrying a crew of two and up to six passengers.

It is believed that the plane was operated by Canadian Air Charters, based in Richmond.

Both police and the TSB are looking for witnesses who may have taken cellphone video of the crash.