The attack on users of the software SolarWinds - which the US has blamed on Russia - was the worst-ever cyber-espionage attack on the US government. The department says 27 US attorneys had at least one office computer hacked.That has raised fears the hackers may have accessed sensitive information, including the names of informants.
"It's potentially very serious," Gil Soffer, a former federal prosecutor, told.
He said prosecutors' emails contain "very sensitive, very confidential and often very secret information".
If the hackers got hold of secret informants' identities, they could use the information to "blow their cover," he added.
The hack, which gave cyber-criminals potential access to 18,000 government and private computer networks, was made public last December. Those hit by the breach include 80% of Microsoft email accounts used by employees at the four New York's attorney offices - which handle some of the most prominent prosecutions in the country.
"There are very sensitive investigations going on within those offices," former federal prosecutor Renato Mariotti said.
They include many high-profile financial investigations, which means any leaked information could be used for blackmail or extortion, he said.
The department says hackers compromised the accounts as early as May 2020 - some seven months before the SolarWinds hack was made public.
The department said all victims had been notified and it is working to mitigate "operational, security and privacy risks" caused by the hack.