The computer IT company that employed the gunman in Monday's deadly rampage at the Washington Navy Yard said a check into his background confirmed he had a national security clearance and "revealed no issues other than one minor traffic violation," company officials said Tuesday.

The computer contractor "The Experts" confirmed in a statement that Aaron Alexis had been employed by the firm for approximately six months over the last year.

He had two tours with "The Experts" as a level-one desktop support employee, a source familiar with Alexis's employment history said.

He was first hired in September 2012 and worked for the company through December of that year. During that stint he spent some time in Japan as part of the company's contract to work on the Navy and Marine Corps intranet system.

He left voluntarily that December with "no issues," the source said, noting Alexis said he was going back to school.

He rejoined the company in late June 2013. During that stretch he worked in various locations of the U.S. as part of the company's contract. Before arriving at the Washington Navy Yard about 10 days ago, he had worked in Stafford, Va., Cherry Point, N.C., Newport, R.I., and Arlington, Va.

Prior to each of his employment stints with "The Experts," the company hired private firms to check the background of the former Navy reservist. The company also contacted the Defense Department and received assurances that his national security clearance was in good standing, the source said.

The background check in late June "revealed no issues other than one minor traffic violation," according to a statement by "The Experts."

A Defense Department official confirmed that Alexis was granted a "secret" national security clearance in March 2008. That level of clearance is valid for 10 years and can carry over to the civilian sector when a service member leaves the military. A Navy official said it is typical for electrician mates like Alexis to receive such a clearance given their work with classified avionics.

It was not this national security clearance that enabled Alexis to enter the Washington Navy Yard on Monday. A Navy official told ABC News that Alexis had a security card known as a Common Access Card, which granted him access to the base.

Alexis received an honorable discharge from the Navy in January 2011 after he applied to leave early. His request was granted after the Navy had failed in seeking a general discharge from the service for what a Navy official termed a "pattern of misconduct." That included eight to 10 incidents, ranging from a traffic violation to unexcused absences and insubordination.

Alexis also went through what's known as a "Captain's Mast," where he may have received a non-judicial punishment for an August 2008 arrest in Dekalb County, Ga., for disorderly conduct.