The FBI is investigating a second wave of threats to Jewish facilities across the US in the last month.
On Wednesday, 27 Jewish community centres in 17 US states reported receiving hoax bomb threats.
The telephone calls prompted evacuations, but no explosives were found or injuries reported.
No one has claimed responsibility for any of the calls, which first targeted 16 Jewish community centres in nine US states on 9 January.
According to the Israeli newspaper Haaretz, the calls were both pre-recorded and live, and the suspect used voice-disguising technology,
The JCC Association of North America, a network of health and education centres, has been reviewing its security plans.
The FBI has not named any suspects or specified a possible motive.
But the law enforcement bureau said that along with the US justice department it was investigating possible civil rights violations in connection with the threats.
"The FBI will collect all available facts and evidence, and will ensure this matter is investigated in a fair, thorough and impartial manner," the statement said.
Paul Goldenberg, national director of the Secure Community Network, an organisation that advises Jewish groups on security, said the sheer volume of threats was startling.
"These are individuals or groups that want to disrupt our way of life," Mr Goldenberg said.
"We're not going to shut down institutions because of this."
The Anti-Defamation League, a Jewish anti-bigotry organisation, said that though the threats did not appear credible, they should still be taken seriously.
The ADL says it has received reports of threats in New York, New Jersey, Ohio, Florida, Massachusetts, Maryland, Michigan, Minnesota, Delaware, Connecticut, Alabama, California, Maine, Tennessee, South Carolina, Missouri, Texas and Kansas.