Omar Mateen alarmed co-workers with claims that he had ties to al-Qaeda and was a member of Hezbollah. One colleague recalled that he frequently used racial slurs and threatened violence while on the job.
Despite those incidents, Mateen never lost his position as a private security guard with one of the world's largest security contractors, nor his license to carry a gun. Now his deadly attack on a gay nightclub in Orlando has raised questions about how the industry screens the guards it entrusts to protect lives and property.
"It is definitely a matter of safety," said Steve Amitay with the National Association of Security Companies, an industry group that supports increased licensing and training standards.
More than 1 million security guards work in the nation's businesses, hospitals, schools and government buildings as of May 2015, according to the Bureau of Labor Statistics.
Mateen, who was 29 when he was killed in a shootout with police, worked for London-based G4S. His most recent assignment was at a residential golfing community in South Florida.
The company said in a statement that it first screened Mateen when he was hired in 2007 and again in 2013 as part of an annual re-screening of a percentage of employees. He was also subjected to a background check by law enforcement.
The screenings included a psychological test, a drug test and checks of his criminal history, employment and education. The process also checked his credit, his Social Security status, his driver's license and his physical fitness. Mateen received a Florida firearms license when he was hired in 2007.
The oversight of security guards varies greatly from state to state, and the inconsistencies have drawn scrutiny in recent years. Many states have systems for licensing, but at least six have no licensing requirements at all.
Regulations can vary even within a state. Denver, for example, has its own licensing requirements. Colorado state government has none.
In New York, security guards must have 47 hours of firearms training before they are certified to carry a weapon, plus eight hours of additional firearms training each year.
As of July 2014, the last date for which nationwide information is available, 15 states did not have any state training requirements for guards regardless of whether they are armed.
Twenty states either do not conduct background checks or check only local and state databases, according to the National Association of Security Companies. The group supports legislation introduced this year in Congress that would allow security firms to obtain FBI checks on employees in states that do not currently conduct any state or local checks.
Roughly 90 bills were introduced in state legislatures last year to toughen state requirements for the licensing and training of security officers. None of the proposals that would have substantially increased requirements was enacted, according to a report by the Pew Charitable Trusts.
Michigan has no state-mandated training requirements, although it does have limitations on who can qualify to be a security guard under a law passed in 1968. Republican state Sen. Darwin Booher has tried for years to update the law, but fellow legislators say they are concerned about creating too much regulation and higher costs for businesses.
Years of discussion have begun to soften the opposition, and Booher expects his legislation will pass when lawmakers return in the fall. The current bill would add several areas of training, including 24 hours of instruction on firearms.
"We put a lot of trust in those uniforms today," Booher said.
The National Association of Security Companies has called for recurring background checks, which would involve constant monitoring of names, Social Security numbers and fingerprints for any arrests. Florida has such a system, but it did not raise any flags about Mateen, who had no criminal record.
The son of Afghan immigrants, Mateen was investigated by the FBI in 2013 over his claims to have terrorist ties. The agency looked at him again in 2014 after learning that he may have had contact with a Florida man involved in an overseas suicide attack. Investigators determined that the two men knew each other only casually and took no action. Mateen was never arrested.
In a statement, his employer said it was aware that Mateen had been questioned by the FBI in 2013, but company officials said they were not told of "any alleged connections between Mateen and terrorist activities."
A former colleague at G4S, Daniel Gilroy, told multiple news outlets that Mateen used gay and racial slurs and regularly made threats of violence, and said he alerted his bosses to Mateen's behavior. Company spokesman Nigel Fairbrass said Wednesday that the G4S had no record of Gilroy complaining about Mateen and that Gilroy has since told them that he never filed a formal complaint.
Fairbass said the company received only one complaint regarding Mateen during his nine years of employment. It came in 2013 from the St. Lucie County Courthouse and prompted Mateen's transfer to another location. That was around the time of the FBI investigation.
The Orlando nightclub shooting is not the first instance of violence involving security guards.
In June 2015, a security guard was charged with killing a woman inside the Iowa mall where they worked. A few months later, an on-duty security guard was charged in the death of a customer at a hospital pharmacy in Virginia following an altercation. He was later convicted of involuntary manslaughter.
Other security guards have been praised for their heroism, including one who died in a shootout with a white supremacist at the Holocaust Memorial Museum in Washington, D.C., and another who was killed in Detroit while trying to save a woman and child from a carjacking in a store parking lot.