As soon as the parcel bomb plot started unfolding, it immediately got shrouded in mystery again. It was initially reported that a 22-years old girl, a medical student, was arrested in Yemen along with her mother for allegedly sending the parcel containing deadly explosive, it was subsequently reported that the cell number of the girl was found written on the parcel invoice which led to her arrest. Her lawyer has said that she has been unwittingly set up. It has now been reported by Reuters quoting a US official that a Saudi bomb-maker believed to be working with al Qaeda‘s Yemen-based wing is a key suspect in the parcel bomb plot against the United States.

Ibrahim Hassan al-Asiri, who tops a Saudi Arabian terrorism list, is the brother of a suicide bomber killed in an attempt to kill Saudi counter-terrorism chief Prince Mohammed bin Nayef last year. That attack, as well as another attempt on a U.S.-bound airliner on Christmas Day 2009, involved the use of pentaerythritol trinitrate (PETN) — a highly potent explosive that appears to be the weapon of choice of al Qaeda’s Yemeni branch, al Qaeda in the Arabian Peninsula (AQAP).

At least one of the two U.S.-bound parcel bombs sent from Yemen addressed to synagogues in Chicago and intercepted in Dubai and Britain on Friday employed PETN. The U.S. official, speaking to Reuters on condition of anonymity, said Asiri was being closely looked at by authorities in view of his experience with explosives. There were also indications he may have been the bomb-maker behind the Christmas Day attempt and the failed attack on Prince Nayaf last year, the official added.

Saudi Arabia, which provided intelligence that helped identify the parcel bomb threat, put Asiri at the top of its terrorism list in 2009. Authorities are scrambling to track down any AQAP operatives behind the latest plot. Yemeni police earlier on Saturday arrested a medical student believed to be in her 20s in Sana’a, but her lawyer told Reuters he feared she had been unwittingly used by others.

U.S. Homeland Security Secretary Janet Napolitano said the parcel bombs sent from Yemen had the hallmarks of al Qaeda, and in particular AQAP. White House counter-terrorism advisor John Brennan has called AQAP “the most active operational franchise” of al Qaeda outside its traditional Pakistani and Afghan base. The Obama administration has been increasingly focused on the al Qaeda wing, which authorities have said was behind the failed attempt to blow up a U.S. jetliner on Christmas Day last year with a bomb that a Nigerian man hid in his underwear.

AQAP is headed by Nasser al-Wahayshi, a Yemeni former associate of Osama bin Laden. But it’s Anwar al-Awlaki, an American Islamist preacher of Yemeni ancestry, who is now drawing considerable attention in Washington. Awlaki, who argues al Qaeda’s extremist views using Western ideas and the Internet, has called the Christmas Day bomber one of his “students” and he traded emails with the U.S. Army psychiatrist who went on a shooting rampage at a military base in Texas last year that killed 13 soldiers. U.S. officials have said Washington has authorized the CIA to kill or capture Awlaki, a rare act against a U.S. citizen that shows the degree of threat he is believed to pose. They have also said the United States will likely increase strikes against al Qaeda in Yemen, seeking to apply the same degree of pressure there as covert drone attacks in Pakistan have had on the core group.