On Monday, shares of G4S, a company that most people have never have heard of, dropped as much as eight percent (nearly $300 million) in trading on the London Stock Exchange.

The company, which is the largest private security firm in the world, boasts over 600,000 employees, one of whom was Omar Mateen, the man who murdered 49 people at a gay nightclub in Orlando on Sunday morning. Mateen had been an employee of the company for nearly nine years, serving most recently as an armed security guard at a gated retirement community in Port St. Lucie, Florida. In the aftermath of the attack, authorities are trying to ascertain how it’s possible that Mateen cleared four background checks, two with the company and two with the FBI, without raising sufficient alarm.

G4S has an unfathomably broad reach. The largest in its industry, it operates across six continents and in hundreds of countries. Mateen was a low-level employee at G4S, but his company has lucrative contracts with countless clients, including the Department of Homeland Security. Profiling its work worldwide for Vanity Fair in 2014, William Langewische noted, “In recent years it has become the third-largest private employer in the world, after Walmart and the Taiwanese manufacturing conglomerate Foxconn. The fact that such a huge private entity is a security company is a symptom of our times.” G4S often provides its security services where industry needs them and where governments can’t or won’t.

Private security firms not only recruit and train guards for small-scale protection, but also provide security (and cybersecurity) infrastructure, investigative services, and military manpower for governments and large industries. Their employees work in prisons and refugee detention centers, as guards at mining sites, nuclear sites, pop shows, and embassy grounds. They also defend airports in conflict zones and fight alongside militias in unstable developing countries.

Not surprisingly, these missions and affiliations open firms up to considerable criticism. GS4, for example, has been denounced in recent years for not training enough security staff for the 2012 London Olympics and for controversial work in Israel and the West Bank, where, according to the Times of Israel, it provided “screening and other services to a number of Israeli organizations, including CCTV cameras installed in Tel Aviv, screening equipment used in the prison system, and security for courts.”

The most notorious private security firm, Blackwater (which no longer exists under that name), was heavily lambasted by those charging that it acted “above the law” while fulfilling U.S. security contracts in Iraq. Meanwhile, in a recent panel for Ars Technica, the UC Davis law professor Elizabeth Joh highlighted a more contemporary concern about private security forces, whic, have the capacity to collect data about consumers at places like department stores without any regulation. 

Even before the number of people displaced by violence reached its highest level since World War II and before the fear of terrorism recently hit its highest point in a decade, the rise of private security firms was seen as an outgrowth of everything from urbanization, the military-industrial complex, and globalization to the rise of tech and a sense of instability in the post-9/11 world.

That unease was kindled anew over the weekend. In a statement on Sunday evening, the North American CEO of G4S emphasized that Mateen “was not on duty” when he attacked the Pulse nightclub in Orlando. One tragic irony is that Mateen had spent years working in a position where it was his job to project a feeling of security and to make sure nothing like this ever happened.