California National Bank, which had struggled with insufficient capital levels for more than a year, was closed late Friday by regulators, marking the fourth bank failure in Los Angeles County this year.

With assets of $7.8 billion, Cal National was the fourth largest bank headquartered in Los Angeles.

U.S. Bank, owned by Minneapolis-based U.S. Bancorp, reached an agreement with the Federal Deposit Insurance Corp. to assume all $6.2 billion of Cal National’s deposits and essentially all of its assets, including nearly 70 branches across Southern California.

Cal National customers will have full access to their accounts over the weekend, the FDIC said. The bank’s locations will reopen Saturday as branches of U.S. Bank.

Cal National’s failure is the largest locally since IndyMac Bank, which had assets in excess of $32 billion, was closed by regulators in July 2008.

Cal National was the largest bank owned by FBOP Corp., a holding company in Oak Park, Ill. with subsidiary banks in four states. On Friday, regulators also closed FBOP’s eight other banks, including San Diego National Bank. The nine banks had combined assets of $19.4 billion.

Regulators said Friday’s closures will cost the FDIC’s deposit-insurance fund an estimated $2.5 billion. The failures bring the 2009 total to 115 across the country.

FBOP, owned by Chicago businessman Michael Kelly, was not closed, regulators said.

The failure of Cal National, which had nearly 1,000 employees, was not entirely unexpected.

The bank has been undercapitalized for more than a year after it was forced to write down half a billion dollars in losses in 2008 on investments in mortgage giants Fannie Mae and Freddie Mac. As of June 30, the most recent date for which data was available, the bank had a total risk-based capital ratio of just 2 percent, well below the 10 percent level needed to be considered “well-capitalized” under regulatory guidelines.

Over the past year, FBOP has tried unsuccessfully to raise capital through measures such as applying to the government’s Troubled Asset Relief Program.

Regulators gave Cal National a deadline of Sept. 30 to raise its capital levels, but the bank was unable to do so. As of Friday morning, FBOP was still trying to find investors for Cal National.

“FBOP is still pursuing capital,” said Cal National Chief Executive Greg Mitchell when reached by phone at his office shortly before noon. “Today is business as usual.”