NEW YORK (AFP) - Lehman Brothers shares plunged further as fears intensified about the survival of the troubled Wall Street investment giant. Lehman shares slid 41.7 percent to end at 4.22 dollars, extending a rout in recent weeks that has erased more than 90 percent of the firm's value. Some said the downward spiral raised the prospect of a collapse in the manner of Wall Street rival Bear Stearns earlier this year. The Washington Post reported late Thursday that the US Treasury and the Federal Reserve are helping Lehman Brothers put itself up for sale. Details were not finalized, but sources familiar with the matter told the newspaper the purchase is expected to be completed and announced this weekend before Asian markets open Monday morning.  A source familiar with the negotiations however said talk of a deal was premature. "It is wrong to imply that there's one single option that is being pursued to the exclusion of all others," said the source, speaking on condition of anonymity. The source added that it was also wrong to imply "that anything is so far developed that it is just 'details to be finalized.'" Treasury Department officials said they were monitoring the situation. "The concern here is that the same thing (that occurred at Bear Stearns) happens with Lehman, that the business deteriorates so quickly ... that they won't be able to stop the collapse of the company," said Mark Pado, analyst at Cantor Fitzgerald. "Lehman's survival as an independent firm looks increasingly in doubt, yet the shape of an end game is not yet clear," said Yves Smith, a financial analyst at the website Naked Capitalism. CNBC television reported that Lehman chief executive Richard Fuld was "actively shopping" for buyers of the venerable Wall Street firm, but that a sale would have to be to a US bank to avoid problems with regulators. The Wall Street Journal said one potential buyer was Bank of America. Pado said it was unclear if a buyer could be found quickly enough. "There is a lot of talk about whether in this environment anybody can raise the money to put in the capital that's needed for a company like Lehman," he said. "It's not just Lehman itself but the deterioration of the financial industry." Another pessimistic view came from Douglas McIntyre of the financial website 24/7 Wall Street. "No one should be surprised if the company goes into Chapter 11 (bankruptcy protection) today or if the government steps in to sell off pieces and guarantee the firm's real estate portfolio to find a buyer," he said. "It may only take one really big client walking out the door." The 158-year-old firm announced plans Wednesday to sell off key assets to shore up its finances as it posted more hefty losses linked to the US subprime real estate crisis. The beleaguered Wall Street firm, seen as in desperate need for a capital injection, lost an estimated 3.9 billion dollars in its fiscal third quarter amid fresh writedowns on mortgage assets. The results, published a week ahead of schedule in view of a near-meltdown in Lehman shares, also noted plans to sharply reduce its exposure to the real estate sector and other steps to raise cash. Standard & Poor's said it kept Lehman on watch for a possible credit downgrade " which if it occurs would make it even more difficult to raise cash. "We continue to be concerned also about Lehman's longer range earnings potential, considering changes in its business mix, potential damage to its business franchise from recent turmoil, and uncertainty as to when market conditions might recover," said S&P analyst Scott Sprinzen. "We continue to view Lehman's near-term liquidity as satisfactory, however." The latest troubles for Lehman come days after the US government took over mortgage finance giants Fannie Mae and Freddie Mac in an effort to stem a global credit crisis. The financial firms have been roiled by the horrific slump in the US real estate market that has battered the banking firms that financed market speculation.