Nine people inside a van fled after it was stopped by a Colorado state trooper Thursday morning near Strasburg.

State troopers, aided by deputies from Arapahoe and Adams counties, rounded up four of them, but still are looking for the other five, said Gilbert Mares, spokesman for the Colorado State Patrol.

The incident began just after 7 a.m. when a state trooper stopped a purple 2000 Toyota van three miles west of Strasburg on Interstate 70, Mares said. It is not known why the van was stopped.

The passengers and the driver "immediately fled on foot," he said. CSP's aircraft helped search the area, which is largely farmland.

"They apprehended four of the nine. We believe the others are at large," Mares said.

Police dogs were helping in the search.

No charges have yet been filed against the four in custody.

Some schools in the area were locked down for a short time, Mares said.