The discovery scuttled plans to remove part of the span, repair the pavement and reopen the freeway before Friday's rush hour. The reopening was pushed to the weekend.
California Department of Transportation officials did not immediately return telephone calls early Friday.
Friday morning's commute was long, slow and unavoidable, said California Highway Patrol Officer Francisco Villalobos.
Volume on alternate freeways was jammed for miles and getting worse. "No matter what you bring along, make sure patience is at the top of the list," Villalobos said.
Demolition work came to a halt when the phone lines were discovered, California Department of Transportation spokesman Patrick Chandler said late Thursday. Workers also discovered a fiber-optic line that, if cut, could disrupt phone service for nearby businesses and homes, he said.
Transit engineers were working with AT&T officials to find a solution.
"This is a challenge and we're going to try to meet it," Chandler said, "but it has caused demolition work to cease at this time."
A double-tanker hauling nearly 9,000 gallons of gasoline went up in flames Wednesday under the Paramount Boulevard bridge in Montebello, east of Los Angeles. The driver was not hurt, but the intense flames and heat melted the truck, cratered the road beneath it and cracked the concrete on the overpass so that chunks crashed onto the pavement below.
Authorities quickly closed the freeway, a major route connecting Los Angeles to dozens of bedroom communities to the city's east. Rush-hour traffic Wednesday evening and Thursday morning quickly became gridlocked for miles, delaying commuters by as many as two hours or more and spilling over onto other freeways and surface streets.
Engineers determined that the damaged steel and concrete made the eastern side of the bridge, built in 1966, too dangerous to save. Concrete core samples were being examined to determine whether the western side would be pulled down as well.
Tens of thousands of drivers use the freeway daily to commute from communities in eastern Los Angeles County and adjoining Riverside and San Bernardino counties. It also is a main route for trucks delivering vast streams of goods from the ports of Los Angeles and Long Beach to inland warehouses.
The shutdown of a key section of Los Angeles area freeway revived memories of the first Carmageddon earlier this year.
In July, a 10-mile stretch of another freeway was closed for a weekend so a bridge could be demolished. Officials had warned for weeks that the work on Interstate 405 in West Los Angeles could create epic traffic jams, however, and people who got the word stayed far away. Traffic in many areas was actually lighter than usual.
That wasn't the case Wednesday.
"We couldn't warn people about this because it was an accident," California Highway Patrol Officer Luis Mendoza said of the tanker fire.
The cause of the fire remained under investigation. There was no crash, so investigators planned to look at other factors, such as possible brake or other mechanical failure.
There were nearly 400 accidents involving tanker trucks hauling chemicals in the U.S. last year and they resulted in five deaths, according to statistics from the U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration.
Most accidents occurred as the tankers were unloading. Only seven occurred in California and none involved a fire.
Taxin reported from Montebello and Jablon reported from Los Angeles.