A St. Paul school bus that had completed its morning run rear-ended a minivan on Interstate 94 Wednesday, setting off a four-car collision that killed a 75-year-old woman and injured six people, authorities said.

The crash occurred shortly before 9:30 a.m. near the Cretin Avenue exit in St. Paul, State Patrol Lt. Eric Roeske said.

The van ended up resting on the nose of the bus.

No children were on the bus at the time of the accident, Roeske said.

Traffic from the east was backed up for at least 3 miles because only one of three lanes remained open.

The bus was moving at about the posted speed limit of 55 miles per hour when it hit the van, which had slowed to about 15 mph because of congestion in the far-right lane, Roeske said.

The van was pushed into an sport-utility vehicle, which in turn hit a car in the rear end, the lieutenant said. The SUV came to rest on its side in the right lane.

Nang T. Nguyen, of Roseville, a passenger in the van and friend of its driver, was killed. She was wearing her seat belt and her airbag deployed.

The driver, Loan K. Nguyen, 41, St. Paul, and four other passengers in the van, all from the east-metro area and ranging in age from 65 to 78, were hospitalized with serious injuries, Roeske said.

The car's driver, a 45-year-old woman from St. Mary's Point, was also taken to a hospital. The bus driver, identified as Tony Thao, 51, of Cottage Grove, was not injured, nor was the driver of the SUV, a 36-year-old St. Paul man, Roeske said.

The bus, operated by Minnesota Central Bus Co., had finished its morning duties and was heading back to its depot, St. Paul schools spokeswoman Toya Stewart Downey said.

Frank Ciccarella, director of safety and training for the bus company's parent, Illinois Central Bus Co. in Joliet, said he is "still trying to piece together what happened. We don't have a definite determination on the cause." Ciccarella added that the bus driver "is OK," but he declined to say any more about him.

Staff writer Nicole Norfleet contributed to this report.

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