"We found him to be the main receiver of the audio clip containing Zawahiri's message," a spokesman for the anti-crime Rapid Action Battalion (RAB) told newsmen hours after 21-year-old Rasel Bin Sattar Khan was nabbed in a pre-dawn raid at his home in central Tangail district.
He said Khan, a textile engineering student, had connections with al-Qaeda and was also found to be running a controversial Facebook page which propagated an Islamist uprising in Bangladesh and was suspected to be a mouthpiece of the fundamentalist Jamaat-e-Islami.
The message, which appeared in local media on February 15, called on Bangladeshi Muslims to launch a jihad against western nations and claimed the country was a victim of plots hatched by Indian and Pakistani elements.
"My Muslim brothers in Bangladesh, I invite you to confront this crusade onslaught against Islam," Zawahiri said in the audio message released by a jihadi website.
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"Bangladesh is the victim of a conspiracy in which the agents of India, the corrupt leadership of Pakistan Army and treacherous power-hungry politicians of Bangladesh and Pakistan are all involved," he was quoted as saying in reports on Bangladeshi news websites.
A day after the message was posted, the Bangladeshi government ordered a probe into its origin saying the country's law-enforcement agencies were coordinating with international organisations to prevent terrorism.
Zawahari became the head of al-Qaeda after Osama bin Laden was killed in a unilateral US military raid on the Pakistani garrison town of Abbottabad in 2011.
His tirade was apparently provoked by Bangladesh's war crimes trials, in which senior leaders of the Jamaat-e-Islami are implicated.