(Recasts; adds color from headquarters, dispute over size of failure, material from conference call, expert comment)

By John Poirier and Rachelle Younglai

WASHINGTON, July 11 (Reuters) - U.S. banking regulators swooped in to seize mortgage lender IndyMac Bancorp Inc IMB.N on Friday after withdrawals by panicked depositors led to the third-largest banking failure in U.S. history.

California-based IndyMac, which specialized in a type of mortgage that often required minimal documents from borrowers, became the fifth U.S. bank to fail this year as a housing bust and credit crunch strain financial institutions.

The federal takeover of IndyMac capped a tumultuous day for U.S. markets that saw stocks slide on a surging oil price and renewed fears about the stability of the top two home financing providers, Fannie Mae (FNM.N: Quote, Profile, Research) and Freddie Mac (FRE.N: Quote, Profile, Research).

IndyMac will reopen fully on Monday as IndyMac Federal Bank under Federal Deposit Insurance Corp supervision, but tensions ran high as customers at a branch at its Los Angeles-area headquarters read a notice in the window saying it was closed.

At another branch down the road, a man who said he had more than $200,000 in an account -- twice what is normally FDIC guaranteed -- argued with a security guard who was closing up.

The FDIC, which will seek a buyer for IndyMac, estimated the cost of the bank's failure to its $53 billion insurance fund at between $4 billion and $8 billion.

"IndyMac is a company that was pretty much 100 percent invested in mortgage assets, and we're in a bad mortgage market, and it had no capital. It's not complicated," said Adam Compton, co-head of global financial stock research at RCM in San Francisco, which manages about $150 billion.