Pressure to overhaul vetting procedures for US government contractors grew on Thursday after one of the largest US security providers admitted that it carried out background checks on Navy Yard gunman Aaron Alexis.

USIS, a Virginia-based company owned by private equity group Providence, had previously denied conducting background investigations into Alexis, according to a report by Bloomberg. Alexis had been arrested three times, including two suspected offences involving guns, although he was not charged or convicted. On Thursday USIS issued a statement in response to mounting questions over how Alexis received the "secret" level clearance that allowed him access to military facilities such as Navy Yard. "Today we were informed that in 2007, USIS conducted a background check of Aaron Alexis for OPM," said a company spokesman in a statement provided to the Guardian. "We are contractually prohibited from retaining case information gathered as part of the background checks we conduct for OPM and therefore are unable to comment further on the nature or scope of this or any other background check."

USIS, formerly known as US Investigations Services, was also involved in background checks on National Security Agency whistleblower Edward Snowden, but subsequently defended its role. In a statement to the Wall Street Journal last month, the company said the federal government did not raise any concerns at the time about its work in February 2011 on the five-year "periodic reinvestigation" of Snowden. The company said the NSA, not USIS, was ultimately responsible for approving or denying Snowden's security clearance.

Snowden's leaks of classified material revealing the extent of the NSA's surveillance activities in the US and abroad prompted a review of vetting procedures for contractors by the Office of the Director of National Intelligence.

On Tuesday, the White House also announced a separate review by the Office of Management and Budget to examine "standards for contractors and employees across federal agencies".