Aamer Madhani, and Kevin Johnson

USA TODAY

There isn’t anything in state or U.S. law that would have prevented the Orlando shooter from legally purchasing the weapons that authorities said he bought just days before he set out to conduct the worst mass shooting in U.S. history, legal experts say.

Federal authorities say that Omar Mateen was armed with a Sig Sauer .223 semi-automatic rifle and a Glock17 that he purchased from a Florida gun store on consecutive days about a week before the rampage. A handgun that authorities believe Mateen may have used in his job as a security guard was also found by law enforcement officials in his car, but was not used in the mass shooting.

Mateen had twice previously been on the FBI’s radar on suspicions of terrorist activity, but was not on the FBI’s large terror watch list database at the time of Sunday’s attack at a gay nightclub that left 49 victims dead and dozens more wounded, according to authorities.

Despite his past brushes with the FBI, the law allowed for Mateen, who worked as an armed security guard, to legally purchase weapons.

Membership or suspicion of membership in a terrorist organization does not prohibit a person from possessing firearms or explosives under current federal law, but the FBI is notified when a firearm or explosives background check involves an individual on the terrorist watchlist, according to the Government Accountability Office. Suspected terrorists purchased more than 2,000 weapons between 2004 and 2014, according to the GAO.

Mateen was the focus of a 10-month investigation in May 2013, while he was working as a contract security guard at a courthouse and told co-workers that he had family connections to Al Qaeda and Hezbollah, FBI Director James Comey said Monday. But Comey, who noted that Mateen was boasting to colleagues of connections to rival terror organizations, said there was no evidence to support that he was committed to the causes he purported to support and the investigation was closed.

The gunman's name surfaced just months later during an inquiry into an American suicide bomber who died in a 2014 attack in Syria, but investigators found no alarming connections between the two. The two went to same Florida mosque, but the FBI found no ties of consequence between the men.

During the first investigation, he was placed on a no-fly list, but was removed after the investigation closed in March 2014, Comey said.

"We are also going to look hard at our own work to see if there is something we should have done differently," Comey told reporters Monday. "So far, I think the honest answer is, 'I don think so.' I don't see anything in reviewing our work that agents should have done differently."

The Senate voted down an amendment in December that would have blocked people on the FBI terror watch list from purchasing guns and explosive materials. The bill included language that would have allowed people who believe they were wrongly placed on the terror suspect list from appealing to the Justice Department.

The National Rifle Association raised concerns that the terror list legislation would lead to Americans wrongly placed on the watch list being denied their constitutional rights to due process. Meanwhile, the American Civil Liberties Union has raised concerns about the veracity of the watch list—noting that government watchdogs have found that as many as 35% of the nominations to the network are outdated and tens of thousands of names are placed on the list without factual basis.

Lindsay Nichols, senior counsel for the Law Center to Prevent Gun Violence, however, said that such legislation is imperative to prevent future attacks.New Jersey is the only state that has a law on the books that prevents a person on the terror watch list from legally purchasing a weapon, she said.

“The law currently defies logic,” Nichols said.

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Mateen’s name was removed from the government’s watch list after the 2014 inquiry concluded. As result, the FBI's Joint Terrorism Task Force would not have been alerted when Mateen sought to buy the weapons in recent days. Had he still been on the list, Mateen’s name would have been forwarded to the task force when he sought to purchase a gun, FBI officials say.

Part of the problem confronting the FBI is that terror watch list too broad and has too many people listed on it, said Michael German, a former FBI special agent. Narrowing constitutional rights based on being placed on a terror watch list, which is believed to include hundreds of thousands of individuals and perhaps 10,000 Americans, is problematic, German said.

"Just having come under investigation just means that somebody made an accusation," said German, who is a fellow with the Brennan Center for Justice’s Liberty and National Security Program . "It doesn't mean that there was any justification for that accusation. The idea that people would lose of their rights or scrutiny or lose some of their rights just because somebody because had made the accusation against them would make the United States a very dangerous place to live."

The incident will also certainly reopen debate about the availability of assault rifles to the public, such as the Sig Sauer .221 that Mateen was armed with. On Monday, presumptive Democratic presidential nominee Hillary Clinton called for a ban on assault weapons, while GOP presumptive nominee Donald Trump said he opposed such a prohibition.

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Critics have blamed assault-weapons, which allow the shooter to rapidly pump fire, for multiplying the death toll in some of the worst mass shootings in U.S. history.

The military-style weapons were once illegal under the federal assault weapons ban, but the ban expired in 2004. Assault weapons sale skyrocketed following the Newtown massacre in 2012, as the Obama administration made an unsuccessful push for legislation that would have placed restrictions on the sale of such weapons.

The National Shooting Sports Foundation estimates that there are currently upwards of 8 million such weapons in the U.S. Seven states and Washington, D.C. currently have some sort of ban on assault-weapons on the books, but Florida is not among them.

The same sort of weapon was used by shooters at massacres in San Bernardino, Calif. where 14 people were killed last year at a holiday party, the Newtown massacre in Connecticut where 26 children and staff were killed, and at a movie theater in Aurora, Colo., where 12 people were killed.

"What happened in Orlando is primarily about the absence of effective gun control laws in the United States," said Jonathan Hafetz, a professor at Seton Hall University Law School. "What other civilized country can a civilian so easily get their hands on a military-style weapon that is designed for use in war? Until we do something about the sale of such weapons, this sort of thing is going to happen again."