"We urge you to strike America in its own home and beyond," says a letter attributed to Ibrahim al-Asiri, the master bomb-maker with al Qaeda in the Arabian Peninsula (AQAP).
The letter, according to a translation by SITE Intelligence Group, states, "America is first."
CNN is unable to independently verify that Asiri himself wrote it, but the letter has drawn the attention of terror trackers such as MEMRI, Flashpoint and SITE.
Another senior AQAP leader, Khaled Batarfi, released a video, which appears to have been shot in a studio, carrying a similar statement. Batarfi does not attribute the message of the video to Asiri, despite its similarities to the letter.
AQAP sprung Batarfi from a Yemeni jail in April.
A U.S. counterterrorism official described the letter as "consistent with rhetoric the new leader stated upon taking over al Qaeda's most active affiliate that is known to threaten Western interests."
A big bounty
Asiri has a $5 million bounty on his head, and analysts say if he did write the letter, he may have been putting himself at risk.
"The concern for Asiri would be that somehow the message would be traceable back to him -- whether by courier, or some digital stamp inside of the message," said Katherine Zimmerman of the American Enterprise Institute. "We have seen U.S. drone strikes kill a series of top al Qaeda leaders in Yemen over the past few months."
But if the letter is genuine, it would indicate that Asiri, who rarely makes public statements, is still alive.
Intelligence officials say Asiri was a key player in the 2009 Christmas Day bomb attempt in which a passenger from Africa almost managed to detonate a bomb aboard a Detroit-bound plane that he'd hidden in his underwear. Asiri was also behind the placing of bombs in printer cartridges aboard planes headed for the United States that were intercepted before they reached their targets.
He even designed a bomb to be carried on the body of his own brother, Abdullah al-Asiri, in attempt to kill Saudi Arabia's counterterrorism chief in 2009. The bomb killed his brother, but the Saudi minister survived.
A dangerous foe
"He's without question the most dangerous terrorist operative that the United States faces today," said CNN terrorism analyst Paul Cruickshank. "Intelligence suggests that he is developing a new generation of explosive devices including a new generation of underwear and shoe-bomb devices."
Zimmerman says Asiri is believed to have taught his skills to a cadre of bomb-makers.
"He has trained a series of individuals who are able to do what he does, which is bring imagination and innovation to an explosive device that could make it through U.S. or Western security," she said.
Batarfi is featured in a second threatening video. In it, he praises the July attack in Chattanooga, Tennessee, in which a gunman killed five American servicemen at a military installation, as "a blessed Jihadi operation." And he also praises two gunmen who tried to mount an attack in Garland, Texas, in May for their "sacrifice and heroism."
"Blood for blood," Batarfi says in a speech posted online.
He then encourages further lone-wolf attacks against America and the West. "To the warriors of Lone Jihad: may Allah bless and guide your efforts," he says.
A U.S. intelligence official said this video is believed to be genuine.
"Batarfi has become a main AQAP media figure since his escape from a Yemeni prison this spring," the official said.
While a number of AQAP leaders have been targeted by strikes this year, the fighting in Yemen between warring factions has deprived the United States of a partner on the ground to work with on tracking and targeting militants.
James Clapper, the director of national intelligence, recently told a conference in Aspen, Colorado, that "in terms of proximate threat, I would view ... AQAP -- even though they're kind of consumed right now with what's going on in Yemen with the Houthis -- as probably our most concerning al Qaeda element in terms of threat to the homeland."