Al Qaeda leader warns of "gravest consequences" if Boston marathon bomber executed

Al Qaeda leader Ayman al-Zawahri warned the United States of the "gravest consequences" if Boston marathon bomber Dzhokhar Tsarnaev or any other Muslim prisoners is executed.

Tsarnaev, named in Zawahri's online video message, was sentenced to death by lethal injection on June 24 last year for the 2013 bomb attack which killed three people and injured more than 260.