Manitoba Agriculture says it did not authorize a crop residue burn Tuesday that has been linked to a three-car highway accident.

A fire in a farmer's field near Elie, west of Winnipeg, spread to a ditch next to the Trans-Canada Highway in the afternoon, the RCMP said, reducing visibility in eastbound lanes to zero.

The accident appears to have happened when one car stopped in the dense smoke and was hit from behind by two other cars, investigators said.

Seven people were injured.

Andrew Nadler, co-ordinator of the Manitoba Agriculture's crop residue burning program, said stubble cannot be burned between Aug. 1 and Nov. 15 unless it is authorized by the department.

"The day in question, yesterday, was not authorized, therefore any burning that did occur in that area was not legal," he said.

"One of the major factors that we look at when we put out authorizations is the atmospheric conditions, so we look at conditions that are appropriate for the dispersion of smoke, and yesterday was not very appropriate," he added.

"We can see that by the amount of smoke that just didn't dissipate. It just kind of stayed low and a lot of it did drift into Winnipeg."

An RCMP spokesperson said police are still investigating the matter.

None of the accident victims' injuries are considered life-threatening.