A teenager confessed last week to causing this month’s Grant Fire, which led the California Highway Patrol to close all lanes of Interstate 580 east of Livermore for hours, authorities said Wednesday.

The July 8 fire, which began next to eastbound I-580 lanes west of the Grant Line Road exit, burned 480 acres of unincorporated county grassland, a total revised downward from an initial estimate of 640 acres.

CalFire crews were joined by firefighters from two counties and at least seven departments in beating back flames. California Highway Patrol officers and Alameda County sheriff’s deputies directed traffic, closing roadways and safely guiding thousands away from and around the scene.

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On July 9, Manteca police called the Alameda County Sheriff’s Office after a local teenager had called to say he and another teen had accidentally started the fire.

After pulling off I-580 about a half-mile east of North Flynn Road in a dirt turnout with car trouble, a 17-year-old pulled out a “safe-and-sane” firework and lit it. The use of such fireworks, which do not leave the ground or explode, is banned in all but a dozen Bay Area communities.

The firework then flew into nearby grass, where dry conditions and winds spread it faster than the pair could extinguish it, Alameda County sheriff’s Sgt. Ray Kelly said Wednesday.

The pair told investigators in a meeting that they called 911 but left the scene soon after, Kelly said.

“They explained in detail what happened and were very cooperative and remorseful for what had occurred,” Kelly said. “They did the right thing. What happened bothered their conscience and they came forward. It’s not a usual occurrence. It’s likely we wouldn’t have figured it out otherwise.”

Investigators have taken the case and will submit it for complaint to the Alameda County district attorney’s office, possibly for charges related to unlawfully causing a fire in a reckless manner.

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Two men wounded in Berkeley shooting Kelly said anyone still in possession of safe-and-sane fireworks can surrender them for disposal at their nearest county fire department, or contact the county bomb squad for different classes of fireworks, Kelly said.

“We’re very fire sensitive in California, particularly in the region the last couple of years. We all have an obligation to protect our communities,” he said. “These fires can start out small and become devastating.”

Contact George Kelly at 408-859-5180.