Terrorist group al-Qaeda has threatened the US of thousands of more deadly attacks, similar to the carnage seen on 11 September, 2001, when terrorists crashed planes into the World Trade Center in Manhattan, New York and the Pentagon.
In a video message, released just before the 15th anniversary of the 9/11 attacks, Ayman al-Zawahiri called the 9/11 attacks a "slap" on the face of the US and its allies.
"We mark in these days the passage of nearly 15 years since the blessed invasions in Washington, New York and Pennsylvania," al-Zawahiri reportedly said in the video.
According to the SITE Intelligence Group's translation of the al-Qaeda leader's message, al-Zawahiri warned the US that "as long as your crimes continue, then the events of Sept. 11 will be repeated a thousand times, Allah permitting".
He also appeared to be taking a dig at rival terror group, the Islamic State (Isis) group, saying that "real soldiers… do not impose themselves on you as rulers without your acceptance and consultation", ABC News reported.
Experts believe that the video release is aimed at pulling the world's attention back to the Islamist group, which was ran by Osama Bin Laden until his death at the hands of a US Navy Seal strike tream. In recent years, al-Qaeda has been overshadowed by the activities of IS (Daesh).
Nada Bakos, a former CIA officer who tracked al-Qaeda in Iraq in the mid-2000s, believe that the video message by al-Zawahiri appeared to partly reflect the "competition" between al-Qaeda and IS.
"All of this [9/11 talk] is very much 'Look what we were able to pull off. No one else can do anything like this'," he said. "He's really trying to present his case [for al-Qaeda]."
Former White House counterterrorism adviser Richard Clarke, who is now a consultant for the counterterrorism analysts, echoed similar views.
"The fact that he [al-Zawahiri] is communicating on the anniversary is an attempt by him to remind everybody that he exists and is the nominal head of [al-Qaeda]," Clarke reportedly said.
He added that the video also showed that the militant group leader has learned from the security mistakes committed by former leader Osama bin Laden, who was killed in 2011 in Abbottabad.
Clarke said that al-Zawahiri has managed to escape an arrest or being killed by the US since the late 1990s, something which bin Laden failed to do. He suspects that the leader is hiding in Pakistan, but most likely keeps changing bases frequently to avoid being tracked.