It’s been clear for a while that merger-and-acquisition activity has been weak in the U.S. But volume in the first quarter has now fallen to the lowest level in four years.

“The slowdown’s key drivers are the credit market’s problems and the economic downturn in the U.S., and potentially global, economies,” Mark Shafir, chairman and global co-head of M&A at Lehman Brothers, told the Financial Times, which reported on the level of inactivity.

Private equity firms, for example, seem to have nearly evaporated in the market due to the credit crunch. In fact, many seem busier trying to rework or walk away from previously agreed upon deals. Indeed, the FT pointed out that deals announced by financial buyers plunged 71 percent, to $52.6 billion, the lowest volume since the first quarter of 2004.

With the credit markets all but shut down, PE firms are unable to obtain financing to do deals.

Meanwhile, strategic buyers, suffering from their own slowdown, seem unwilling to step up and buy one of their rivals. That was the case this weekend, in fact, when Aloha Airlines unable to find a qualified buyer or win financing to keep aloft.” decided to shut down its interisland and transpacific passenger flight operations,

In addition, Maguire Properties Inc., a small owner of office properties in Los Angeles, today said it is no longer actively trying to sell the company, as previously announced. It cited “current market conditions, particularly in the credit markets, and the lack of any viable acquisition proposal received from third parties.”

Meanwhile, the FT reported that Bank of America may back away from its plan to sell its equities prime brokerage unit after receiving lukewarm interest from potential bidders.