(CNN) -- The Navy was unaware until after the killings at the Washington Navy Yard that gunman Aaron Alexis had been arrested in 2004 for shooting out the tires on a vehicle and claiming he had blacked out in anger, a senior Navy official told reporters Monday.

However, the government agency that carried out Alexis' background check in 2007 did know that he omitted information about that brush with the law but granted him security clearance anyway after Alexis explained it.

The Navy said it didn't learn the details of the incident until after Alexis killed 12 people last week at the sizable naval installation in Washington.

In 2007, contractor USIS conducted the security clearance assessment for Alexis, who had just joined the Navy.

A report for the Navy from the Office of Personnel Management, which processes and grants security clearances, didn't mention a gun, the Navy official said.

It said only that Alexis had "deflated the tires on a construction worker's vehicle" in the 2004 incident, which occurred in Seattle.

According to the Office of Personnel Management, the agency uncovered Alexis' arrest for malicious mischief through a check of FBI criminal history records.

The Seattle Police Department would not make records available, and the agency tried to corroborate the arrest through court records.

The Navy said Alexis answered "No" to questions on his security clearance form about whether he had ever been arrested or charged with a felony. Alexis told the security clearance personnel he did not disclose the malicious mischief charge because he was told it was going to be dismissed and removed from his record.

The search of the court records corroborated Alexis' version, the Office of Personnel Management said, but these records did not mention that a weapon was used. During a follow-up interview, Alexis did not mention having shot the tires out.

"Had the Seattle Police Department permitted OPM to obtain the law enforcement records, it is possible that additional information may have been obtained," the agency said in a statement.

Alexis, 34, was a subcontractor last week when he entered the Navy Yard and the shooting rampage began. In addition to the 12 deaths, Alexis wounded eight people before he was killed.

A woman who was wounded in the shooting was released from a hospital Monday. Only one person remains hospitalized -- a police officer who is listed in good condition.

USIS is the same contractor that processed NSA leaker Edward Snowden's clearance and is now under investigation.