The outlook for US life insurers dimmed badly Friday after an across-the-board downgrade from a key ratings agency sent stocks of major companies in the industry plunging by 20 percent or more. Standard & Poor's Ratings Services late Thursday lowered its ratings on several US life insurers and life insurance holding companies, saying the troubled economy is putting increased pressure on their assets. S&P lowered its credit and financial strength ratings on 10 groups of US life insurers, and its credit ratings on seven US. life insurance holding companies. Among them were major players like MetLife Inc. Hartford Financial Services Group Inc., Prudential Financial Inc. and Conseco Inc.