WASHINGTON (CNN) -

Ramping up his investigation of the White House and whether it illegally used government staff for political purposes, House Oversight Chairman Darrell Issa released audio of a 2012 voicemail message that his office says depicts then-Labor Secretary Hilda Solis asking an employee working for her to contribute money and recruit others to attend a fundraiser for the pro-Obama Organizing For America.

In a release, the committee called the phone message a clear violation of the Hatch Act, which limits political activity for government officials.

"I'm not in a position to comment on that audio recording," White House spokesman Josh Earnest told reporters when asked about it. "It relates to an ongoing law enforcement investigation."

Solis, who won election to the Los Angeles Board of Supervisors in June, did not respond to CNN's request for comment.

In the voice message, a woman identifies herself as Hilda Solis and then says, "Wanted to ask you if you could, help us get folks organized to come to a fundraiser that we're doing for Organizing for America for Obama campaign on Friday. ... There are a lot of folks that we know that are coming but wanted to ask you if you might help contribute or get other folks to help out."

A Republican source told CNN the message was left on March 20, 2012, and that the Office of Special Counsel, which enforces the Hatch Act, received a complaint just under a month later.

According to a letter from the OSC to Solis that was obtained by CNN from the same Republican source, it opened an investigation of the matter but ended it without drawing any final conclusions in January 2013 because Solis left her Cabinet position.

"In conformance with OSC's policy not to continue investigations once an employee leaves federal service, OSC will close its investigation of this matter," the letter reads.

The White House did not indicate which branch of government was still investigating the matter, but the Los Angeles Times reported in May that the FBI had questioned Solis and a few of her associates and that at least one witness had also testified before a grand jury.

Issa, a California Republican, had subpoenaed White House political director David Simas to testify at a Wednesday hearing. Simas did not attend after the White House balked at the request, arguing that separation of powers between the branches means that he cannot be compelled to testify.

The White House insists it has not done anything inappropriate.

Rep. Ellijah Cummings of Maryland, the top Democrat on the Oversight panel, has said Issa has abused his power in his investigation, repeatedly issuing subpoenas without consulting the full committee.