-- tonight that US special forces moved in. And captured the so called mastermind of the attack and been -- four Americans were killed -- that a tap tonight a suspect in custody taken down in a secret... See More
-- tonight that US special forces moved in. And captured the so called mastermind of the attack and been -- four Americans were killed -- that a tap tonight a suspect in custody taken down in a secret nighttime raid. ABC's chief investigative correspondent Brian Ross to back with the surveillance they used to catch him. The US had quickly determine the -- -- believe was one of the masterminds of the massive deadly attacks and then. But it was not until now 21 months later that the US was able to capture Ahmed Abu -- Identified today by US officials as this man the leader of an Islamist terror group in Libya. Yesterday. Our special forces. Showing incredible courage and precision. We're able to capture an individual. -- -- Who was. Who is alleged to have been one of the -- -- of the attack. US officials tell ABC news that could tell ahead but under surveillance in -- for a -- In fact he was interviewed by reporters for CNN and others denying his involvement and flaunting his presence and then got right -- If CNN reporter can -- it. Interview with him openly in -- cafe. Why couldn't we did to him sooner. US officials say a plan to capture Abu could tell -- was delayed after public outrage in the deal over this video of the takedown of another accused terrorist in Tripoli last October. Today the mother of one of the Americans killed -- then Ghazi told our San Diego affiliate KG TV her questions about what happened are still not answered. I'm glad they got one of them. There was a whole bunch and set fire to that place. Officials say -- look at -- is now being held on a US -- in the Mediterranean where he's being interrogated before being sent to the US for trial. With the key questions being who ordered the attacks and where are the others who -- involved. As the president said the US will find those responsible David no matter how long it takes all right Brian Ross -- is here tonight Brian thank you.
This transcript has been automatically generated and may not be 100% accurate.