A driver who may have suffered a heart attack lost control of his big rig on the 405 Freeway during the height of the Wednesday afternoon rush on Los Angeles' Westside, leading to an eight-car pile-up, authorities said.

[Updated at 5:25 p.m.: After initial reports that 15 vehicles were involved in the crash, Los Angeles Fire Department officials reduced the number to eight. California Highway Patrol officials estimated there were a dozen.]

The number of injuries was not immediately known, but the CHP confirmed that paramedics were treating a man for a medical emergency, which sources familiar with the incident believe may have been a heart attack.

The pile-up was reported about 4:30 p.m. on the northbound 405 just north of its junction with the 10 Freeway. It was touched off by what CHP officials described as a "runaway truck."

The CHP said several of the northbound lanes of the 405 were closed.

-- Andrew Blankstein