Los Alamos, N.M. ? The director of the Los Alamos National Laboratory was replaced Thursday amid a growing number of government investigations into charges of widespread theft and fraud at the birthplace of the atomic bomb.

John Browne, a physicist and lab veteran who became director in 1997, will step down Monday.

Browne, 60, said he was not pressured to quit by federal officials or the University of California, which runs the lab for the Energy Department. But he said university President Richard Atkinson accepted his resignation during a Dec. 23 conversation in which Atkinson said the lab may need “management change” to address its problems.

“The controversy was so strong and so critical of management that I personally thought the best thing for me to do was resign and to have the university come in and take it to the next level of performance,” Browne said in an interview.

Energy Secretary Spencer Abraham, who met with Browne last month, said: “It is crucial that we restore public confidence in the management of the laboratory.”

The FBI, Energy Department and at least two congressional committees are looking into alleged credit card abuses at the lab during the past several years and the disappearance of high-tech hardware and other equipment.

Two lab-hired investigators were fired in November and they went public, alleging a cover-up of the wrongdoing. The lab gave no reason for the dismissals and has not replaced the two men.

“They continue to cover up and to conceal the situation at the lab, and finally they’ve had to pay for what they’ve done,” said Steven Doran, one of the fired investigators.

The lab also has been tarnished by security scandals, including missing computer disks and the controversy involving former scientist Wen Ho Lee, who was jailed for nine months after being accused of stealing nuclear secrets. He denied any wrongdoing and ended up pleading guilty to a single felony count after the government’s case crumbled.

The university, which has managed the lab since its inception in 1943, appointed retired Navy Vice Admiral George Nanos as interim director.

John Browne, director of the Los Alamos National Laboratory, who is shown in this March 2002 file photo, has resigned. Browne's resignation was announced Thursday and comes in the midst of government investigations of fraud and theft at the New Mexico laboratory.