Four people died in a major traffic wreck immediately west of where Interstate 90 crosses Minnesota Highway 13 shortly after 2:30 p.m. Wednesday.

Traffic had been restricted to two lanes on the westbound side of I-90 because of construction on the eastbound lanes.

According to witnesses who were driving other vehicles and the Minnesota State Patrol, a woman driving an eastbound 1996 Toyota Corolla swerved in front of a westbound semi. The driver of that semi, Randy Pedersen of Swaledale, Iowa, said the crash damaged his steering, causing his semi to cross into the oncoming traffic.

The truck hit a 1996 Honda Odyssey minivan head-on. The semi continued through the muddy ditch and onto the new and unused pavement of eastbound I-90, where it stopped.

All three passengers of the Honda died. The woman driving the Toyota also died.

Pedersen said he couldn’t understand why a driver would steer right in front of his truck, but a witness who had been in a Dodge Ram pickup just behind the Honda speculated the Toyota driver might have suffered a medical occurrence.

The Toyota and the minivan were totaled. Pedersen, 56, was unharmed. The front of the semi was severely damaged. His load on a flatbed trailer was not damaged.

The names and ages of the deceased are expected to be released later today. A reconstruction team was called to the site to piece together how the crash took place.

Diesel, gasoline and oil seeped into the ditches. Both Interstate 90 eastbound and westbound lanes near Minnesota were closed due to a semi crash, according to the Minnesota Department of Transportation. Traffic was backed up in both directions.

The I-90 overpass of Highway 13 remained closed for several hours until maintenance vehicles removed wreckage and the team could reconstruct the crash. Traffic was routed to the off-ramp of Exit 154, across Highway 13, with the help of officers at the intersection, and back onto the on-ramp to I-90.

It is the deadliest automobile collision of 2009 and likely the worst of recent years.