Trading was halted yesterday in shares of the lender Thornburg Mortgage Asset after they plunged 47 percent, the most since their 1993 initial public offering. Thornburg joined others singing the mortgage blues. Earlier, five brokerage firms downgraded the stock and Moody’s Investors Service cut its debt rating amid turmoil in the home loan market.

Brokerage firms cited concerns that Thornburg, which specializes in high-quality, prime jumbo mortgages, might need to sell assets or reduce its dividend because of a liquidity squeeze.