LOS ANGELES (AP) — Federal regulators have indefinitely delayed a decision on the proposed restart of the shuttered San Onofre nuclear power plant in California, raising new questions Monday about whether the twin reactors will produce electricity again.

The seaside plant between San Diego and Los Angeles has been dark since January 2012, after a small radiation leak led to the discovery of unusual damage to hundreds of tubes that carry radioactive water.

Operator Southern California Edison wants permission to restart the Unit 2 reactor and run it at reduced power in hopes of stopping vibration and friction that was blamed for damaging tubing.

The Nuclear Regulatory Commission delayed several earlier target dates for a ruling, with officials recently projecting a June announcement. But its website on Monday listed no date for a restart decision — only "to be determined."

Agency spokesman Victor Dricks had no comment.

Last week, the NRC's Atomic Safety and Licensing Board sided with environmentalists who have called for lengthy hearings on the restart plan after concluding that firing up the plant would allow Edison "to operate beyond the scope of its existing license."

A statement from SCE spokeswoman Jennifer Manfre noted that NRC Chair Allison Macfarlane indicated earlier that no decision would be made until at least mid-June on the company's request to change its operating license to run at lower power, a critical step in the restart plan.

"SCE continues to adhere to the established regulatory process," the statement said. The company "cannot restart Unit 2 until the NRC says that it is safe to do so."

Last month, SCE's parent, Edison International, raised the possibility of retiring the plant if it can't get one reactor running later this year. The company also disclosed that costs tied to the long-running shutdown had hit $553 million.

Edison is facing a tangle of regulatory obstacles that include a separate state investigation into who should pay for the trouble — customers or shareholders.

Meanwhile, anti-nuclear activists and some lawmakers have said restarting the plant would lead to a disaster.

Friends of the Earth, an advocacy group challenging the restart, believes no decision can be made "until all the safety issues raised by the board are addressed," spokesman Shaun Burnie said in an email.

Even with San Onofre sidelined, state power officials predict that there should be adequate power supplies in California this summer, but heat waves or wildfires that damage transmission lines could lead to potential shortages.

The problems at San Onofre center on steam generators that were installed during a $670 million overhaul in 2009 and 2010. After the plant was shut down, tests found some generator tubes were so badly eroded that they could fail and possibly release radiation, a stunning finding inside the nearly new equipment.

San Onofre is owned by SCE, San Diego Gas & Electric and the city of Riverside.