Just days after clashing sharply with Republican lawmakers over his agency’s political woes, Internal Revenue Service Commissioner John Koskinen said Wednesday he remained an “optimist” despite multiple investigations demanding more information into suspected IRS targeting of conservative and tea party groups for special scrutiny.

“It’s illusory to think that we’ll never have a problem or make a mistake, that things will always go the way we’ve planned,” Mr. Koskinen told a National Press Club luncheon. “Instead, my goal has been for us to find problems quickly, fix them promptly, make sure they stay fixed and, perhaps, most important be transparent about the entire process.”

House Republicans exploded last week when Mr. Koskinen — who was not at the agency when the suspected targeting first took place — said it will take years to fully comply with requests for information and records in the scandal. The commissioner acknowledged Wednesday that the targeting charges meant that “doubt has been cast by some on the independence of the IRS.”

Mr. Koskinen’s remarks come at a busy time for the IRS. Not only are six investigations underway over possibly politicized audits, but tax season is in full gear, and the IRS is set to play a critical role in enforcing the insurance mandate at the heart of President Obama’s Affordable Care Act.