The Justice Department is investigating the loss of Internal Revenue Service emails that might shed light on the agency's treatment of conservative groups.

As part of its criminal probe into the IRS's treatment of politically active conservative groups, the Justice Department is "investigating the circumstances of the lost emails from [former IRS official Lois Lerner's] computer," according to prepared testimony by James Cole, the deputy attorney general. Mr. Cole is set to appear at a hearing scheduled for Thursday...