(Updates with third fatality)

By Dana Ford

LOS ANGELES, Oct 13 (Reuters) - Three people were killed and at least 10 injured in a multi-vehicle crash that caused a fire inside a tunnel and forced the closure of a major freeway in the Los Angeles area, authorities said on Saturday.

The fire broke out late on Friday, following a traffic pileup involving as many as 15 trucks inside a tunnel under Interstate 5. The highway connects Los Angeles to many of its northern suburbs and San Francisco.

By Saturday afternoon, firefighters had extinguished the flames, but officials said it could be days before the crucial roadway, located about 30 miles (48 km) north of downtown Los Angeles, was reopened to traffic.

The southbound lanes of the freeway might reopen on Tuesday "if things progress the way they are right now," Jason Hurd, a spokesman with the Los Angeles County Fire Department, said.

He declined comment on the northbound lanes, although adding it was highly doubtful either direction would open before Monday.

Engineers are on scene to determine the severity of the damage to the structure of the tunnel. Based on their assessment, 10 of the 12 lanes of the highway could remain closed to traffic indefinitely.

Officials worry there could be major traffic bottlenecks if the road remains closed at Monday's start of the work week, said Officer Anthony Martin of the California Highway Patrol.

"We're doing everything that we can at this time to get that portion of the freeway back open," Martin said.

The 10 people taken to hospitals suffered various injuries, including burns, but nothing life-threatening.

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