"For this, I deeply apologize," said Juan Thompson, 32, who also pleaded guilty to a charge of conveying false information and hoaxes.
Federal prosecutors said Thompson sometimes used his girlfriend's name while making threats against Jewish community centers, schools or other facilities.
They said one message claimed he had placed two bombs in a Jewish school and was "eager for Jewish Newtown," a reference to the 2012 school massacre in Connecticut.
The government collected evidence from about two dozen laptops, tablets and cellphones seized from his home.
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Thompson was fired from the online publication The Intercept last year after being accused of fabricating story details.
Since January 9, there have been more than 150 bomb threats against Jewish community centers and day schools in 37 US states and two Canadian provinces, according to the Anti- Defamation League, a Jewish group that battles anti-Semitism.
The threats led to evacuations and sent a chill through local Jewish communities. Acts of vandalism on Jewish targets, including cemeteries, have added to those concerns.