The United States said Suleiman Abu Ghaith, a son-in-law of Osama bin Laden, should spend the rest of his life in prison on Monday after he is convicted on terrorism-related charges, according to The Associated Press.

Abu Ghaith, 48, had been convicted in March by a Manhattan federal jury for conspiring to kill Americans, conspiring to provide material support for terrorists, and providing such support, the AP reported.

Abu Ghaith is a Kuwaiti-born imam who married bin Laden's eldest daughter about five years ago and is the highest-ranking al-Qaida figure brought to trial on U.S. soil since the 2001 attacks, according to the AP.

In a court filing, prosecutors portrayed Abu Ghaith as a charismatic mouthpiece for al Qaeda after the Sept. 11, 2001 attacks who recorded inflammatory videos to recruit new members, and said the defendant showed no remorse and lied at his trial, according to the AP.

Prosecutors also rejected arguments by defense lawyers, who seek a 15-year prison term, that Abu Ghaith did not knowingly plot to murder Americans, and that his speeches amounted to mere "whistling past the graveyard" because al Qaeda was already reeling, the AP reported.

"The defendant played a central role in al Qaeda's conspiracy to kill Americans, unapologetically celebrating the murder of nearly 3,000 people, proudly proclaiming to the world that this senseless tragedy was justified and America deserved to be attacked, boldly threatening more attacks of similar magnitude, and working to drive more fighters to join al Qaeda's deadly mission," prosecutors said, according to the AP.

"A sentence of life imprisonment is the only appropriate sentence in this case," they added, the AP reported.

The sentencing is scheduled for Sept. 23 before U.S. District Judge Lewis Kaplan in Manhattan, and Abu Ghaith is set to testify, offering a glimpse into bin Laden's activity shortly after the Sept. 11 attacks, according to the AP.