Explosions were heard in the skies over Qatar, which is home to a major US military base.

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WASHINGTON - US government officials are increasingly concerned that Iran or its allied militias could retaliate for bombings of Iranian nuclear facilities with attacks inside the country.

In an internal email Sunday, senior FBI officials warned that Iran and its proxies “have historically targeted U.S. interests in response to geopolitical events, and will likely increase their efforts in the near term.”

Officials asked FBI offices around the country to monitor their sources and maintain close contact with the Defense Department, including the National Guard. The email also called for “specific attention” to U.S. military installations linked to the attacks on Iran.

In New York City, Mayor Eric Adams said more police would be on duty near religious, cultural and diplomatic sites “out of an abundance of caution” given the situation in the Middle East.

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Iran, which the U.S. has designated a state sponsor of terrorism, has long supported a network of militias across the Middle East in an attempt to extend its influence across the region and undermine Israel. Those militias include Hamas, Hezbollah and the Houthis, who control parts of Yemen.

In parallel, the US Department of Homeland Security said in a bulletin that the “ongoing conflict in Iran” raised national security concerns and that cyberattacks by pro-Iran hackers were likely.

“The likelihood of violent extremists in the country independently mobilizing for violence in response to the conflict would likely increase if the Iranian leadership were to issue a religious ruling calling for retaliatory violence against targets in the country,” the bulletin said.

The Department of Homeland Security also noted that “cyber actors affiliated with the Iranian government may carry out attacks against U.S. networks.”

Such warnings are traditional after U.S. military actions abroad and reflect national security officials' concern that citizens living in the United States could become angry or emboldened to act violently.

Relations between Iran and the United States have become more tense in recent years, especially after American officials accused Tehran of trying to assassinate U.S. officials and political dissidents.

At a meeting on Sunday, Homeland Security and FBI officials warned state and local law enforcement agencies about the threat to American communities. During the meeting, the nonprofit Secure Community Network, which provides security consulting and training to Jewish facilities across North America, said the threat level was very high.

“Iran would not attack the United States unless a red line was crossed,” Michael Masters, the group’s executive director, told the meeting. “The red line was assessed as direct military engagement, especially targeting Iran’s nuclear facilities. That red line has now been crossed.”

The Department of Homeland Security bulletin said the war between Iran and Israel “could also motivate violent extremists and perpetrators of hate crimes who seek to attack targets perceived as Jewish, pro-Israel, or linked to the U.S. government or military in the country.”

Former FBI officials said the agency was well acquainted with situations like this and would ask informants to uncover new threats while also monitoring existing cases, including possible surveillance of suspects.

In the email sent Sunday, the FBI said it should prioritize leads associated with Iran or its proxies.

According to Carlos Fernandez, a former senior FBI agent in charge of the New York counterterrorism division, agents need to take the possibility of sleeper cells in the United States seriously, especially since Iran has been accused of plotting to kill President Donald Trump before the election and a human rights activist in Brooklyn.

Indeed, the department has also uncovered Hezbollah members who trained in Lebanon but later moved to the U.S., where they were arrested in Michigan and New York and charged with terrorism. “It’s very real,” he said. “It’s a legitimate concern.”