SAN FRANCISCO (CBS SF) — Lanes of Interstate Highway 280 were closed Tuesday afternoon following a fatal incident involving a pedestrian, the California Highway Patrol reported.

The incident was reported at about 4:30 p.m. Witnesses reported seeing a person fall onto the highway near the Monterey Blvd. exit.

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At least two southbound lanes were closed and traffic heavily backed up on both southbound 280 and southbound U.S. Highway 101.

Officers from the San Francisco Police Department and the CHP were investigating the incident as a suicide.

The San Francisco Fire Department said two off-duty nurses saw the person tumble onto the freeway, but did not see if the person jumped.

The nurses started CPR, but the person was dead at the scene, the fire department said.

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