The threat of sleeper cells in the U.S. has “never been higher,” though there are no current specific threats, according to U.S. Customs and Border Protection (CBP).

A memo sent Saturday from CBP Commissioner Rodney Scott, obtained by The Hill’s sister network NewsNation, said “thousands of Iranian nationals have been documented entering the United States illegally and countless more were likely in the known and unknown got-a-ways.”

“Though we have not received any specific credible threats to share with you all currently, the threat of sleeper cells or sympathizers acting on their own, or at the behest of Iran has never been higher,” Scott added.

The memo urged CPB personnel to remain “vigilant.”

On Saturday, Trump announced that the U.S. had bombed three Iranian nuclear sites, stepping into an ongoing conflict between Iran and Israel. Iran’s ambassador to the United Nations slammed the U.S.’s involvement in its conflict against Israel and said his nation’s military would figure out its response following Trump’s ordering of the bombing of the nuclear sites.

NewsNation correspondent Rich McHugh said Friday on the network that “experts that we’ve talked to said that should the U.S. join these strikes against Iran, they may try to activate these sleeper cells in the U.S. and the West.”

On Sunday, retired Gen. Frank McKenzie, the former commander of U.S. Central Command (Centcom), said that he believed U.S. troops in Iraq and Syria were “certainly vulnerable” to retaliatory attacks by Iran.