“I don’t know who the hell they think they are to not share that information with us,” Representative Matt Gaetz, a Republican from the Panhandle, said in a news conference on Thursday after the Florida delegation met behind closed doors in the Capitol with officials from the F.B.I. and the Homeland Security Department.

Now that they know, Florida officials are prohibited from sharing the details with voters.

Adding to the lawmakers’ anger were worries about what the hackers did once they gained access to the voter rolls. F.B.I. officials told lawmakers that they had no evidence any data had been altered, but they could not say “with certainty” that no manipulation had occurred, said Representative Debbie Mucarsel-Powell, a Miami Democrat.

The voter registration systems are separate from voter tabulation systems, which officials say were untouched. Voter rolls are public information in Florida, but access to the registration data could have allowed the hackers to delete or add voters, cancel their mail-in ballots or alter their party affiliation.

“We still have a lot of questions,” Ms. Mucarsel-Powell said.

Tucked in the Mueller report last month was confirmation that the F.B.I. had determined that a Russian military intelligence unit known as the G.R.U. had breached “at least one Florida county government” during the 2016 election.

That revelation prompted members of Congress to request Thursday’s briefing. Its importance only grew after Gov. Ron DeSantis, a Republican, disclosed on Tuesday, after his own briefing, that two counties had been hacked. The governor added a note of exasperation that the F.B.I. had required him to sign a nondisclosure agreement to receive the classified information.