J.P. Morgan & Co.'s performance in investment banking and its $3 billion share buyback plan prompted a debt downgrade by Standard & Poor's, a ratings agency analyst said Tuesday.

"J.P. Morgan has strong niches but it isn't as strong and broad-based as Merrill (Lynch & Co.)'s, whose rating had been lower before the downgrade," said S&P analyst Tanya Azarchs. "If they are looking to become an investment bank, we want to see them become a leader."

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