An autopsy was pending on Saturday into the cause of death of an 18-year-old man who was killed when he lost control of his bicycle and crashed into a wall on the UCLA campus in Westwood, officials said.

The rider died at UCLA Medical Center, where he was taken after the crash at 9:50 p.m. Friday on De Nave Drive and Charles E. Young Drive, said Los Angeles Police Sgt. C. Clark of the West Traffic Division, which was investigating the accident.

Jerico Culata of Los Angeles was participating in the Critical Mass ride with up to 700 other riders when he lost control of his bike and slammed into a masonry wall, Clark said. He wasn't wearing a helmet, said LAPD Officer Sara Faden.

Faden couldn't confirm reports about whether the bike Culata was riding had brakes. She did say, though that there is a trend of riders are using bikes that have no brakes.

Culata went wide on a downhill curve, his friend told a photographer outside the hospital.

Several other riders also crashed, but none was seriously hurt, according to Culata's friend.

Police in recent years have added patrols around the rides to quell vandalism and other crimes committed along the route, once even escorting riders along the streets in an event in 2010, according to the LAPD website.

A watch commander at the LAPD's West Traffic Division said that extra motorcycle patrols and traffic collision investigators were trailing the ride on Friday.

There was trouble reported at the Critical Mass cycling event in San Diego on Friday night. A San Diego police officer was recovering after being hit in the head with a screwdriver that a rider allegedly threw.

It was unclear if any of the riders were arrested near Sixth Avenue and Market Street downtown, where the crime occurred about 9:10 p.m. Friday. The officer was taken to a hospital with a bleeding head wound, Officer David Stafford told the U-T San Diego.

The last-Friday-of-the-month ride, held in about 300 cities worldwide, is an outgrowth of the Critical Mass movement started in San Francisco in 1992.

The name of the ride refers to the number of riders needed to dominate streets.

In Southern California, the riders can sometime be rowdy. In other cases, the riders don't follow the rules of the road.

For instance, “corking” is when a handful of the leaders block an intersection to enable the larger pack of riders to pass through.

Run-ins with police during Critical Mass rides are not uncommon, since some riders apparently embrace the night time tours as a form of protest.

Some of the riders are on bikes with no brakes, said a rider who participated in the Friday night event in Westwood. It was not immediately known whether the teen who died was on a bike with no brakes.

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