WASHINGTON: Omar Mateen , the lone gun man, who killed 50 people at a gay nightclub in Orlando today was under the radar of US law enforcement agencies in the past, media reports said."He (Mateen) is a known quantity. He's been on the radar before," a law enforcement source was quoted as saying by The Daily Beast.According to the media report, Mateen became a person of interest in 2013 and again in 2014.At some point, the FBI also opened an investigation into Mateen but subsequently closed the case when it produced nothing that appeared to warrant further investigation, the source said.Meanwhile, the Florida Governor Rock Scott described this as an act of terror."This is an act of terror," Scott told reporters at a news conference in Orlando.Mateen's father Mir Siddique has issued an apology for the mass shooting."We are saying we are apologising for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country," he told NBC news channel.