Top brass at the Internal Revenue Service violated agency policy -- and possibly federal law -- by repeatedly using non-secure private email accounts to conduct official business, including sending classified documents to non-agency email addresses, says the head of the House oversight panel.

House Oversight and Government Reform Committee Chairman Darrel Issa, R-Calif., in a letter to IRS Acting Commissioner Daniel Werfel obtained by the Washington Examiner, outlined a "troubling pattern" involving at least four senior IRS officials that raises concerns that taxpayer privacy protections were breached.

"This not only raises the prospect of violations of the Federal Records Act but it also raises data security concerns and violates internal IRS policies," Issa wrote.

He said he discovered the emails while investigating the ongoing IRS scandal in which the agency targeted conservative and tea party groups that were applying for tax-exempt status during the 2012 election season.

One of the officials Issa names is Lois Lerner, who was at the center of the scandal. His investigation accuses her of producing more than 1,600 pages of emails and documents housed in a nonofficial email account related to official business, including almost 30 pages of confidential taxpayer information. Among the material was a summary of an application for tax-exempt status the IRS instructed Lerner's legal counsel to redact because the information was prohibited from being disclosed.

Lerner retired last month amid an ongoing internal probe of the scandal.

Issa's investigation also showed that former IRS Commissioner Douglas Shulman, who stepped down after the November election, also received emails related to official business on a nonofficial email account.

Other IRS officials Issa targets are Judith Kindell, senior technical adviser of the Exempt Organization Division, and Nikole Flax, chief of staff of the Office of the Commissioner.

"This rampant use of nonofficial email by four IRS officials to conduct official business suggests that such use is a systemic problem throughout the IRS," Issa wrote. "This is also a concern to the committee because federal taxpayer information cannot be shared on nonsecure, nonofficial systems."

The Republican has requested Werfel brief the committee regarding his agency's handing of emails.

Because of the partial government shutdown, a recording at IRS offices said agency spokespeople were unavailable to comment.

The Environmental Protection Agency also came under fire this year for using private email accounts for official business. But the agency's inspector general last month cleared top-level employees of intentionally trying to hide information by using private email, saying the problem involved a failure to train employees on how to properly store records.