A probe conducted by the Massachusetts Secretary of State revealed evidence that suggests CSFB research analysts rated stocks favourably in order to keep clients and secure new ones.
Massachusetts officials said they reviewed company e-mails that showed CSFB analysts were under pressure to make positive ratings assessments about internet stocks in the high-flying 1990s.
CSFB analysts allegedly gave stocks a "buy" rating while secretly telling its larger clients and insiders to steer clear of them.
Such allegations led to a $100m (£64.2m) settlement in May between New York Attorney General Eliot Spitzer and Merrill Lynch, the leading US investment bank.
Further investigations
In conducting their investigation, Massachusetts officials reviewed thousands of electronic messages and other documents and talked with CSFB employees.
Reports suggest executives at CSFB client firms were granted special access to newly floated shares of stock.
Massachusetts Attorney General William Gavin is pondering whether to hand the case over to criminal prosecutors.
He has referred the case to Mr Spitzer, who has vast powers to bring charges of wrongdoing or criminal actions.
Last month, US regulators suspended and fined two senior executives at CSFB for allegedly ramping up brokerage fees during the dot.com boom.
CFSB said it had taken "internal disciplinary action" against the employees involved after settling the investigation.
The allegations of tainted ratings are the same charges that led to the dismissal of star Merrill Lynch analyst Henry Blodget, who gained fame in the late 1990s for his bullish ratings of dot.com stocks.
Mr Spitzer's office is amidst an investigation into the actions of analysts at Salomon Smith Barney, a unit of Citigroup.
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