The leak began in October in a vast underground storage field in Porter Ranch, on the outskirts of Los Angeles.
The natural gas leak, one of the largest in US history, has caused thousands to relocate.
The escaping gas at the Aliso Canyon storage field accounted for a fifth of all methane emissions - an extremely potent greenhouse gas - across the entire state of California.
"This has been among the worst environmental catastrophes in recent history," said Robin Greenwald, a senior attorney with Weitz & Luxenberg, a law firm which is taking legal action on behalf of some Porter Ranch residents.
"The public health crisis it caused is far from over," said Ms Greenwald, arguing that her clients still did not know if it was safe to return to their homes.
They were uncertain about the safety of the 114 other wells on the site and had not been given "assurances that the interior of their homes are safe," she said.
"The lack of transparency by all those involved is inexcusable."
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