New York prosecutors are saying a life prison sentence is the only answer for the son-in-law of Osama bin Laden after he was convicted for his role as al-Qaida’s spokesman after the Sept. 11 attacks. In papers submitted Monday, prosecutors said a life sentence for Sulaiman Abu Ghaith sends an important deterrent message to would-be terrorists. Abu Ghaith was convicted in March of charges that included conspiracy to kill Americans and providing support to al-Qaida. Abu Ghaith is a Kuwaiti-born imam who married bin Laden’s eldest daughter about five years ago. He is the highest-ranking al-Qaida figure brought to trial on U.S. soil since the 2001 attacks. Sentencing is set for Sept. 23. Earlier this month, defense lawyer Stanley Cohen recommended a 15-year sentence.