(U.S. Senate)-In the aftermath of the Orlando shooting that left 49 people dead and dozens injured, Senator Jon Tester is demanding answers from the U.S. Department of Homeland Security (DHS) on the background investigation and screening process that the shooter was subject to as an employee of a government contractor.

In a letter to DHS Secretary Jeh Johnson, Tester pointed to "troubling signs" in shooter Omar Mateen's background screening investigation and outlined specific concerns with the level of oversight the DHS has over contract employees. Mateen was employed as private security guard by G4S-an international security company contracted by the federal government. He twice passed the company's background screening process that provided him access to state firearm and security licenses.

"As the investigations into this tragedy continue, it is imperative that we also ensure that depraved individuals like Mateen are properly vetted and barred from these sensitive posts," Tester wrote. "It is clear that the screening procedures in place failed in this case. It is critically important that we all take the necessary steps to ensure we are keeping our nation safe by ensuring that individuals charged with the protection of others are suitable, stable, and capable of fulfilling their duties."

Tester called on Secretary Johnson to release more details on the background screening process at G4S, the oversight mechanisms that DHS has in place to ensure contract employees are properly screened, and whether Mateen could ever have worked at a federal facility as an employee of G4S.

G4S recently received a $234 million contract from DHS, in the past with the State Department, Justice Department, Energy Department, Drug Enforcement Administration, U.S. Army, and U.S. Air Force.

In 2007, Mateen was fired as a security guard from a Florida State Prison and then hired as a contractor for G4S later that year.

Tester, the former Chairman of the Subcommittee on Efficiency and Effectiveness of Federal Programs and the Federal Workforce, held a series of oversight hearings on the background investigation process last Congress. As a result, he introduced and passed into law the SCORE Act which allows for more resources to investigate cases where the background check process may have been compromised.

Tester has also introduced the SCARE Act to prohibit federal contractors and employees who have compromised the integrity of a background investigation from performing security clearance investigations in the future. This bill unanimously passed the Senate Homeland Security Committee six months ago and is awaiting a vote in the Senate. You can read all about Tester's work reforming the background investigation process HERE.

The day after the Orlando attack, Tester renewed his call for strengthening American's national security.

Tester's letter to Secretary Johnson is available HERE.