The passengers in a minivan that crashed on an icy U.S.interstate during the weekend's winter storm, killing seven, were illegal immigrants being smuggled to North Carolina, a federal immigration officer confirmed Tuesday.

"This was a smuggling vehicle," said Carl Rusnok, a spokesman for the U.S. Immigration and Customs Enforcement's Dallas office. "These people were being smuggled and transported illegally.

"The question now is whether there is a larger smuggling operation behind this particular vehicle."

Seven people, including the driver, died Sunday when the 1998 Chevrolet Astro van hit a patch of ice along Interstate 40 in Oklahoma, slid across the center median and slammed into an oncoming tractor-trailer. The crash happened about 3:50 a.m., when there was limited visibility and freezing rain in the area.

The van was heading from Nogales, Arizona, to North Carolina, along what Rusnok described as a common smuggling route.

The Oklahoma Highway Patrol has not released the names of those killed in the crash.

Five other passengers were transported to the Great Plains Regional Medical Center in Elk City, Oklahoma. At least one of the survivors was released and taken into federal custody, Rusnok said.

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