BlackRock, a large U.S. investment company, is expected to be the main asset manager for the $75 billion fund being created by three large banks to help shore up the market for asset-backed securities, according to a person briefed on the situation.

A custodian for the fund could also be appointed shortly, according to people with knowledge of the matter, and the Bank of New York Mellon is a leading contender for that role.

No official announcements have been made on either position. But with plans under way to begin raising, as early as next week, more than $60 billion for the fund, there is a heightened sense of urgency to fill both roles.

The three biggest banks in the United States - Bank of America, Citigroup and JPMorgan Chase - settled on the structure of the proposed fund about a week ago, following two months of negotiations held against the backdrop of deteriorating economic conditions in the United States.

U.S. banks and government officials are hoping that the backup fund will allow the financial instruments known as structured investment vehicles, or SIVs, to sell in an orderly fashion the more than $250 billion in assets they hold and to avoid severe disruption to already jittery credit markets.

If, as expected, BlackRock is appointed manager, it would be responsible for managing the securities sold into the fund's portfolio, deciding whether to sell an asset at a particular price or hold it to maturity. It is not clear whether BlackRock would determine those prices or if they would be set by an outside party.

BlackRock, known for its keen attention to risk, would lend both credibility and capital markets expertise to a proposal that has been viewed from the start as shaky.

The company recently indicated that it had less than 1 percent of its cash assets invested in SIV-related debt, and it has largely steered clear of the subprime mortgage investments that have recently spread distress throughout the financial industry.

BlackRock was chosen over at least two other investment management companies for the right to handle the assets of the backup fund, according to people briefed on the matter, even though Laurence Fink, founder and chief executive of BlackRock, had previously had reservations about the company's participation in the fund.

Brian Beades, a BlackRock spokesman, declined to comment.

The bank chosen for the custodian assignment would oversee the financial record-keeping and other back-office operations for the new fund. Should Bank of New York Mellon get that assignment, it would be a prominent and potentially lucrative task for the bank, which has largely shunned the spotlight despite its legacy as the oldest bank in the United States, founded 223 years ago.

Kevin Heine, a spokesman for Bank of New York Mellon, declined to comment.

How much importance the backup fund, a so-called super-SIV, might have is unclear; many investors and analysts are skeptical that it would be able to have a large effect on current market conditions.

Debt market conditions have rapidly deteriorated over the past few weeks as investor appetites for pools of assets - from mortgages to auto and, more recently, credit card loans - have all but dried up.

Those behind the creation of the fund hope that it can buy enough time for asset prices to recover, although most market analysts say that is improbable. The fund could, however, discourage SIVs from dumping their holdings all at once, a move that could cause securities prices to collapse.

That would force banks to take even bigger write-offs and could also put pressure on money market funds, whose exposure to SIV-issued commercial paper could put them in jeopardy.

Bank of America, Citigroup and JPMorgan Chase are each expected to put $5 billion to $10 billion into the fund. The remaining $60 billion or so is expected to be raised from dozens of financial institutions, including several outside the United States.

Bank of America and JPMorgan are expected to lead the fund-raising effort. Citigroup will take a more minor role, people involved with the plan said, because it could disproportionately benefit if the plan provides relief to the seven troubled SIVs it operates.

Officials at Bank of America, JPMorgan and Citigroup either did not return calls seeking comment or could not be reached.