U.S. Department of Homeland Security Secretary Jeh Johnson is shown speaking Feb. 13, 2015. On Friday, Johnson expressed support for those mourning losses as a result of terrorist attacks in France, Kuwait and Tunisia, and encouraged Americans to "remain vigilant" during Fourth of July weekend. Reuters/Robert Galbraith

The Department of Homeland Security and the FBI were warning law enforcement Friday about the possibility of terrorist threats around the Fourth of July, unnamed authorities told CNN Friday. Officials circulated a joint intelligence bulletin that does not discuss "any known active threats," but does sound the alert about the potential for "heightened threats," CNN reported.

Separately, DHS Secretary Jeh Johnson issued a public statement, expressing support for victims of the terrorist attacks that struck France, Tunisia and Kuwait on Friday, while also urging vigilance in the U.S. "Particularly with the upcoming July Fourth holiday, here in the United States the Department of Homeland Security and the FBI continue to communicate with state and local law enforcement about what we know and see," Johnson stated. "We are encouraging all law enforcement to be vigilant and prepared."

Johnson said that "all Americans" should continue to "attend public events and celebrate."

Three separate terrorist attacks within hours of each other Friday killed dozens of people abroad. The victims include at least 37 people left dead by a gunman at a resort in Tunisia, and at least 27 people killed in a suicide bombing at a Kuwait mosque. The first attack occurred at a U.S.-owned gas facility in France, where authorities discovered one victim who was beheaded.

Although the Islamic State group, also known as ISIS, claimed responsibility for only the Kuwait bombing, the other two attacks shared similarities with previous strikes by ISIS, raising the possibility of a coordinated strategy to coincide with the Muslim holy month of Ramadan, which began June 17, International Business Times reported Friday.

ISIS expert Andrew Tabler of the Washington Institute for Near East Policy told IBTimes earlier: "This is an indication of the way of the future, where you have multiple lone wolf attacks in multiple countries at the same time." Tabler added that it is hard to defend against that type of attack.

According to the CNN report, federal law enforcement officials have stepped up arrests of suspected extremists in recent weeks, and the FBI has increased its surveillance of some ISIS suspects.