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WASHINGTON (CNN) -- The deaths of alleged terrorists Anwar al-Awlaki and Samir Khan could spark others to launch attacks against the United States in retaliation, according to the FBI and Department of Homeland Security.

The agencies issued a joint intelligence bulletin late Friday to state and local law enforcement partners.

The bulletin said supporters might seek to portray al-Awlaki as a martyr in a supposed U.S. war against Islam. The document, obtained by CNN, says the death of al-Awlaki and Khan "could provide motivation for Homeland attacks" by "homegrown violent extremists," the type the two men allegedly tried to recruit or inspire. The document also says members of Al Qaeda in the Arabian Peninsula could be motivated to strike.

The law enforcement bulletin says officials have no information indicating anyone is planning attacks.

However, initial reports of al-Awlaki's death "generated a relatively high level of interest among users of Web sites with violent extremist content, who mourned" him and hope he will be viewed as a martyr. Some of those communicating on the Internet also expressed doubts al-Awlaki had really been killed and are seeking confirmation.

The bulletin lists a number of instances in which al-Awlaki communicated with or inspired attackers.

The most notable involve alleged underwear bomber Umar Farouk Abdulmutallab, who is facing trial in the U.S. for allegedly attempting to blow up a Detroit-bound plane on Christmas of 2009. The document issued Friday says al-Awlaki allegedly "provided instructions on detonating an explosive device" to Abdulmutallab. It also says al-Awlaki "directed" the attempted cargo bomb plot in 2010.

Law enforcement views lone wolves as extremely hard to detect because they are not working with co-conspirators, and it may be hard to see the signs an attack is coming. So the bulletin asks local law enforcement to be vigilant for unusual activity.

After Osama bin Laden was killed in a raid by Navy Seals, U.S. officials issued a bulletin warning his supporters might strike in retaliation.