The FBI on Thursday said it has received "credible information of a broad threat to synagogues" in New Jersey.
The agency's Newark office asked the public to take all possible security precautions as it investigated shortly after 3 p.m.
"We are taking a proactive measure with this warning while investigative processes are carried out," the FBI said in a subsequent tweet.
New Jersey Gov. Phil Murphy said he was working with law enforcement to ensure houses of worship throughout the state are protected.
Attorney General Matthew J. Platkin said his office was working with federal and state authorities to coordinate a response.
"Law enforcement will be increasing patrols in sensitive areas. Some of these patrols will be in marked vehicles and others will not – but please do not be alarmed if you observe an increase in police presence as we are taking these steps in an abundance of caution," Platkin said in a statement. "We also call upon New Jersey residents to be extra vigilant and if you see suspicious activity please report it to your local authorities without delay. We continue to work together to keep New Jersey safe, and let those who would bring harm to the residents of this state know that New Jersey will not be home to hate, intolerance or violence."
No additional information was immediately provided. The FBI said it would share more information when it could and urged anyone in an emergency situation to call the police.