The source could not give more detail and Fitch could not be reached.
The raid by police on Fitch comes after a raid last week in the Milan officers of another major ratings agency, Standard & Poor’s.
Prosecutors in the southern Italian town of Trani accuse the world’s three main ratings agencies S&P, Fitch and Moody’s of market abuses in an investigation begun in 2010 following a complaint by consumer groups.
Magistrates accuse Fitch of hinting that Italy’s sovereign credit rating could be downgraded by 31 January. The investigation focuses on the negative effect of credit ratings on the share valuations of Italian banks.