"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.
RAB officials said Khan would be brought before the media later today. 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.
Zawahari became the head of al Qaeda after Osama bin Laden was killed.
"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.
"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.
The message claimed the onslaught was being orchestrated by "leading criminals in the subcontinent and the West".
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.