Douglas Shulman, a former Bush appointee, told the Senate Finance Committee that he learned all the facts about what happened when he read last week's report by a Treasury inspector general.
"I agree this is an issue that when someone spotted it, they should have brought it up the chain. And they didn't. I don't know why," Shulman said during Congress' second hearing on the scandal.
Shulman vacated his position last November when his five-year term expired, and this was his first public remarks since the story broke.
When committee Chairman Max Baucus, D-Mont., asked how the improper screening system could have occurred in the first place, Shulman responded, "Mr. Chairman, I can't say. I can't say that I know that answer."
He said he first learned about the targeting and the inspector general's investigation in the spring of 2012, during the presidential election. He said that in a meeting with outgoing acting commissioner Steven Miller, he was told that IRS workers were using a list to help decide which groups seeking tax-exempt status should get special attention, that the term "tea party" was on that list and that the problem was being addressed.
Asked by Sen. John Cornyn, R-Texas, whether he owed conservative groups an apology, Shulman said, "I'm certainly not personally responsible for creating a list that had inappropriate criteria on it."
"I very much regret that it happened and that it happened on my watch," Shulman said.
For more than a year, from 2011 through the 2012 election, members of Congress repeatedly asked Shulman about complaints from tea party groups that they were being harassed by the IRS. Shulman's responses, usually relayed by a deputy, did not acknowledge that agents had ever targeted tea party groups for special scrutiny.
At one House hearing on March 22, 2012, Shulman was adamant in his denials, saying, "There's absolutely no targeting."
At a separate hearing, Treasury Secretary Jacob Lew said the IRS's actions against conservative groups were "unacceptable and inexcusable." Lew said he first learned about the inspector general's investigation in March but that he was unaware of the findings until they became public this month. Lew became Treasury secretary in February and was White House chief of staff before that.
In a reversal from previous claims, the Obama Administration now admits that the IRS did inform a top White House lawyer last month about its investigation into improper scrutiny of conservative groups. But according to the administration, that lawyer stopped short of sharing any information with President Barack Obama.
Throughout the escalating firestorm, the White House had maintained that it first learned about the scandal through press reports - a notion that Republicans said they found highly suspect.
The Associated Press contributed to this report.