LINCOLN, Neb. — Nebraska's attorney general sued 16 state lawmakers on Tuesday to prevent the corrections director from having to testify before a committee about his department's lethal injection protocol.

Attorney General Doug Peterson filed the lawsuit on behalf of Nebraska Department of Correctional Services Director Scott Frakes.

The lawsuit seeks to block a subpoena issued by the Legislature's Judiciary Committee following a complaint by state Sen. Ernie Chambers, of Omaha, a fervent death penalty opponent.

Attorneys general have filed similar lawsuits in the past in cases that involve separation-of-powers disputes between the state's executive and legislative branches, but such moves are unusual.

"It has happened, but it's rare," said Clerk of the Legislature Patrick O'Donnell.

O'Donnell, who also was named in the lawsuit, said the Legislature will likely hire an outside attorney to defend him and all of the affected lawmakers.

The Nebraska attorney general is generally responsible for representing state officials in legal actions, but state law allows the Legislature to hire its own lawyer in matters such as Tuesday's lawsuit. Lawmakers didn't consult the attorney general's office before they issued the subpoena, said Suzanne Gage, a spokeswoman for Peterson. Peterson, a Republican, supports capital punishment.

"The Attorney General took the action of filing the lawsuit in order to uphold the law, which is his duty," Gage said in an email.

Frakes is an appointee of Republican Gov. Pete Ricketts, but his department has faced intense scrutiny from lawmakers because of persistent overcrowding and staffing problems and a series of high-profile incidents, including two riots and an escape in the last three years. Chambers and other committee members want to know more details about how the department selected the drugs it hopes to use on at least two death-row inmates.

Chambers contends the department's new execution protocol is unconstitutional and has questioned whether state officials followed the correct procedures for putting it into place. Frakes was ordered to appear before the committee at a public hearing on May 8.

The complaint came as state officials try to begin the process required to execute two of Nebraska's 11 death row inmates. The Department of Correctional Services faces several lawsuits from civil liberties groups over the protocol and its refusal to release records that would identify its supplier of lethal injection drugs — information the department released in the past without objections. Legislation that would have given the department the authority to withhold such information stalled in the 2017 session.

Nebraska's last execution took place in 1997.

The lawsuit in Lancaster County District Court alleges that the subpoena to force Frakes to testify violates Nebraska's constitutional separation of powers. Peterson asked a judge to quash the subpoena and bar lawmakers from taking similar actions in the future.

It also contends that Frakes was directed to testify before the wrong committee. The Judiciary Committee, which includes Chambers, has been known to grill Frakes in past hearings. The lawsuit argues that the Government, Military and Veterans Affairs Committee should have jurisdiction.

The lawmakers named in the lawsuit are members of the Judiciary Committee and Executive Board, even though some members voted against calling a hearing or issuing the subpoena. The Executive Board's permission was needed to issue the subpoena, and three of its members voted against doing so.

Sen. Laura Ebke, the chairwoman of the Judiciary Committee and one of the defendants, said she had just received a copy of the lawsuit and couldn't comment. Ebke said the committee was still planning to hold its hearing.

"We're reviewing our options," she said.

Sen. Dan Watermeier, the Executive Board's chairman, said he had expected the attorney general to challenge the subpoena.

Watermeier said the question of issuing the subpoena was limited to whether it was frivolous and part of the committee's jurisdiction. He said he didn't consider it frivolous and believes the Judiciary Committee was the appropriate committee to handle it.

Chambers didn't immediately respond to a phone message left at his home.