Two children are improving while one remains in critical condition after the car they were riding in slid into a near-freezing pond in suburban Minneapolis in a crash that has left two of their siblings dead, authorities and the family's attorney said.

The mother and stepmother of the children was at the wheel with only a learner's permit, which requires a licensed driver to be in the car with her, the Minnesota State Patrol said.

The car veered off a rain-slicked highway ramp on Thursday and plunged into a retention pond in St. Louis Park. The five children were trapped inside the vehicle as it sank into water up to 2.7 metres deep and remained there for up to 45 minutes, police said.

Alarious Coleman-Guerrido, 7, and Zenavia Rennie, 5, died on Thursday, the state patrol said. Three other young passengers — Aliyana Rennie, age 1; Zarihana Rennie, 6; and Amani Coleman-Guerrido, 5 — were injured in the crash.

Attorney Rick Petry, who is speaking for the family, said Friday two of the children are improving and now responding to health workers' commands. He calls that "a huge improvement."

Petry says a third child remains in stable but critical condition.

The driver, Marion Guerrido, 23, escaped uninjured, according to police.

'Never quite prepared'

Guerrido, who lives in Brooklyn Center, Minnesota, was dropping the children off at her mother's house before heading to work when the crash happened an hour before sunrise, Petry said. Her husband was also on his way to work at the time.

"They are just like the typical American family," the attorney said. "They were starting their day."

The four older children attended Odyssey Academy in Brooklyn Center, where grief counsellors were on hand Friday to help other students and staff, executive director John Sedey said, calling the crash a devastating blow for the school.

"You are never quite prepared for this kind of thing," Sedey said.

The children were unresponsive after being pulled from the 1998 Pontiac Grand Am, which slid 12 metres down an embankment into a retention pond, said Jamie Zwilling, spokesman for St. Louis Park, a suburb west of Minneapolis.

Guerrido escaped and screamed for help, but the children were trapped inside as the car sank, said State Patrol Lt. Eric Roeske. Passers-by unsuccessfully tried to rescue the children before police arrived.

The investigation into the crash is likely to take several weeks.

"We have not identified anything or ruled out anything at this time, other than alcohol," Roeske said, adding that there was no evidence Guerrido had been drinking.

"We have received no information nor have encountered any evidence that would indicate it was an intentional act, but the investigation continues.

Police were investigating what caused the car to go off the ramp, which does not have guardrails, and whether the children were in seat belts or car seats, Roeske said.

With files from The Associated Press