Islamabad, Pakistan ? The killing of an al-Qaida chemical weapons expert in a missile strike two weeks ago on a Pakistani border village has dealt a heavy blow to the terrorist group’s ambitions to build weapons of mass destruction, a former CIA case officer says.

Abu Khabab al-Masri was dubbed by terrorism analysts as al-Qaida’s “mad scientist.” His most notorious work, recorded on videotape, showed dogs being killed in poison gas experiments in Afghanistan when the Taliban ruled.

“If he is out of the picture, al-Qaida’s weapons of mass destruction capability has been set back, which would make this one of the more effective strikes in recent years,” Arthur Keller, an ex-CIA case officer in Pakistan, told The Associated Press. Keller led the hunt for al-Masri in 2006.

The U.S offered a $5 million bounty for the 55-year-old Egyptian, and the CIA had been hunting him for years. Al-Qaida confirmed his death days after the July 28 attack by unmanned drones on a tribesman’s compound in the village of Azam Warsak in South Waziristan.

Al-Masri, whose real name is Midhat Mursi al-Sayid Umar, got his chemical weapons training in the Egyptian army before defecting to the militant Islamic Jihad group, founded by al-Qaida’s no. 2 leader, Ayman al-Zawahri.

The U.S. government says that since 1999, al-Masri had been distributing manuals for making chemical and biological weapons.

“I believe that al-Qaida has no shortage of people adept with explosives, and I know that al-Masri promulgated training manuals for poisons,” Keller said, “but I’m not sure how skilled any of Al-Masri’s proteges may be at synthesizing chemical weapons or toxins.”

It’s not easy, he said.

“You need both education and hands-on experience to produce decent-quality chemical weapons or toxins.”

Chlorine has been used in bombings by militants in Iraq, but these were locally inspired, a U.S. counterterrorism official said on condition of anonymity, not being authorized to comment publicly on the sensitive issue.

Also, no evidence has surfaced that al-Masri continued the chemicals research after moving to Pakistan, although the U.S. government said he was likely carrying out training.