OLYMPIA — Expressing concern about an ongoing state investigation into the early release of thousands of prisoners in Washington state over a 13-year period, Republican leaders of a Senate committee said Tuesday they are seeking subpoena power in order to conduct their own investigation.

Senate Law and Justice Committee Chairman Mike Padden and Vice Chairman Steve O’Ban, joined by Senate Majority Leader Mark Schoesler, told reporters at a news conference that they had concerns about the independence of the investigation currently being done by two retired federal prosecutors. Democratic Gov. Jay Inslee hired the investigators to lead a probe into why a software coding error that miscalculated prison sentences occurred and then went unfixed for so long.

The error, disclosed Dec. 22, has led to the early release of up to 3,200 prisoners since 2002. At least two deaths have been tied to the early releases.

“Let’s be clear about what’s going on here. This is the case of the governor as the chief executive of our state who oversees the Department of Corrections investigating himself,” said O’Ban, R-Tacoma. “We are in this unfortunate position as a co-equal branch of government that has the duty, the responsibility, to provide accountability to the administrative branch.”

Padden and O’Ban notified Democratic Inslee of their intentions in a letter late Wednesday night, though Inslee on Thursday noted that the press was notified before his office was.

Inslee told reporters that if lawmakers had specific questions, all they had to do was to ask him or his staff.

“We all want to get to the bottom of this,” he said, and stressed the independence of the investigators’ work.

“I am not controlling or directing this investigation, and that’s important,” Inslee said, saying that implications otherwise are trying “to make this some kind of political partisan thing.”

“If you ask people who is more likely to get to the bottom of this fastest, it’s probably two prosecutors instead of one political party in an election year,” he said.

The Department of Corrections was first alerted to the error — which started in 2002 — in December 2012, when a victim’s family learned of a prisoner’s imminent release. The family did its own calculations and found he was being credited with too much time.

However, even though the agency consulted with attorneys regarding the error the same month and scheduled a fix for the program, it was repeatedly delayed and ultimately, never done. Department of Corrections Secretary Dan Pacholke said he didn’t learn of the error until the middle of last month, and the governor says he didn’t learn of the issue until that same time, when corrections’ officials notified his staff.

The committee’s move comes after Padden, R-Spokane Valley, said too many unanswered questions remained following a committee hearing earlier this week. At the Monday hearing, Pacholke was unable to answer several of Padden’s questions, including why it took more than a month for the head of the agency’s technology department to notify senior officials in about the problem, even though Padden said his questions for Pacholke were submitted in advance. Padden also said that lawmakers’ efforts to obtain records from the governor’s office and the Department of Corrections have been delayed.

Inslee spokeswoman Jaime Smith wrote in an email that the requests were being “handled properly and in accordance with the public records act.” She wrote that neither Padden nor his staff has requested one of the documents cited in his letter, a contract between the state and the investigators, but the office would have “been happy to give it to him if he had asked our office for it.”

“From the start, our administration has cooperated fully with the Legislature and we will continue to do that,” Smith wrote.

The Senate committee plans to start the subpoena process next week by passing a resolution seeking all documents from the Department of Corrections and the governor’s office pertaining to the erroneous early releases.

“This is something that was thrust upon us and we have an obligation to get to the bottom of it,” Padden said.

According to state records, the last time a legislative committee sought subpoena power was in 1988, when the Senate Rules committee authorized the Senate Law and Justice Committee to get records regarding alleged child abuse by a superior court judge that were held by the state Judicial Conduct Commission. The state Supreme Court quashed the subpoena, saying that that legislative subpoena power could not be used to compel violation of the commission’s confidentiality rules.