Police on Thursday arrested the driver of a van involved in a crash with a school bus that killed four students in rural Minnesota.

Alianiss N. Morales was being held on suspicion of criminal vehicular operation, the Lyon County sheriff's office said.

The charge alleges that "driving conduct contributed to the death of another human being," said Lt. Brian West of the Minnesota State Patrol. There are six levels of the charge under Minnesota law, and all but one are felonies.

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County Attorney Rick Maes said he hoped to file formal charges against Morales on Friday after reviewing additional reports from law enforcement, including witness statements.

Two brothers, the daughter of a teacher and the son of a former teacher at Lakeview School in Cottonwood were killed in Tuesday's crash about two hours southwest of Minneapolis.

At least 14 other people were hurt; many remained hospitalized Thursday.

Morales was driving a van east on County Road 24, which has a stop sign as it ends in a T intersection with state Highway 23, the road on which the bus was traveling, authorities said.

The van struck the bus, which rolled over onto a pickup truck, police said.

West said he couldn't say whether Morales ran the stop sign and wouldn't discuss the circumstances of the crash.

"We've been examining as much information as we can, but we're not able to discuss that at this point," he said.

The driver of the truck told the Star Tribune of Minneapolis that he saw the van run the stop sign.

"I saw the maroon van run a stop sign, it blew over the railroad tracks and hit the bus, sending it sliding into me," James M. Hancock, 45, of Marshall, told the newspaper from his hospital bed in Sioux Falls, S.D. "The next thing I knew, they were hauling kids out and it was chaos."

Hancock did not want to give interviews Thursday, said Kenyon Gleason, a spokesman for Avera McKennan Hospital.

Before her arrest, Morales could not be reached for comment and did not have a listed phone number.

West declined to comment on whether Morales, 23, had a valid driver's license. She was convicted in 2006 of driving without a valid driver's license, according to court records.

Classes at Lakeview School resumed Thursday, and a memorial service was planned that night in the gymnasium. Gov. Tim Pawlenty and most of Cottonwood's 1,150 residents were expected to attend.

Seven students and two adults injured in the crash remained hospitalized Thursday at nearby Avera Marshall Regional Medical Center, the Sioux Falls hospital and the Mayo Clinic in Rochester, but all were improving, said Mary Maertens, CEO of the Marshall hospital. Five were in fair condition, two were in serious condition and two were in stable condition, she said.