House Oversight and Government Reform Committee Chairman Rep. Darrell Issa, R-Calif., subpoenaed the Environmental Protection Agency on Friday seeking documents related to the agency's email practices.

The committee has questioned the EPA's use of separate, internal email accounts after discovering former administrator Lisa Jackson maintained an account under the alias "Richard Windsor."

Issa said the agency has not adequately responded to an April request for documents he made with Sen. David Vitter, R-La., the top Republican on the Senate Environment and Public Works Committee. He requested correspondence between the EPA and the White House regarding the requests made by the committee.

"Given the EPA's continued lack of cooperation, I am left with no choice but to issue a subpoena to compel the production of documents relevant to the Committee's oversight obligations," Issa wrote in a letter to EPA Administrator Gina McCarthy.

The Republican lawmakers, with conservative and government ethics groups, contend the use of private email accounts stretches beyond Jackson, who now works for Apple.

They argue EPA officials could be conducting official business through email addresses in an attempt to shield such communication from public records requests.

The EPA, which has agreed to review its email protocol, has defended the use of the separate accounts.

It says it would hand over all correspondence from those email addresses in a public records request, noting the practice dates to former President George W. Bush's administration.

Issa, though, said the agency is taking too long to hand over committee-requested files.

In Friday's letter, he cited a new example that indicated a senior attorney with the EPA had emailed an assistant to President Obama saying that the agency was prepared to hand over 106 documents in response to Issa and Vitter's April request.

But those documents never arrived, Issa said.

"Despite EPA's apparent desire to fulfill their obligation and turn over these documents to the committee, they were never produced," Issa wrote. "I am left with the impression that EPA's lack of cooperation results from a coordinated effort to keep information from the committee and the American public."

"We will review the subpoena," EPA spokeswoman Alisha Johnson said in an email.