SAN FRANCISCO >> For an aspiring prosecutor, a recent job posting for a special assistant U.S. attorney in Sacramento might appear perfect at first glance.

It only required one year of legal experience, promised training in investigating and trying federal crimes and would put the successful applicant just a short hop from San Francisco’s night life and Lake Tahoe’s recreation.

But there was a catch: No pay.

U.S. attorneys’ offices around the country have been offering such unpaid jobs for at least the last six years or so, luring candidates with the prospect of an invaluable work experience or the chance to perform public service. Assistant U.S. attorney positions are prestigious and can boost careers in government or the private sector.

But critics say the unpaid jobs threaten racial diversity in federal prosecutors’ offices and set a bad precedent for labor standards.

“Only wealthy people can afford to do this,” said Rory Little, the former head of the appellate unit at the U.S. Attorney’s office in San Francisco who now teaches at the University of California, Hastings College of the Law. “It’s sad really. It’s like any indentured servitude.”

An association made up of assistant U.S. attorneys goes a step further, saying the unpaid positions violate the law.

Supporters point to the short duration of the unpaid jobs — most appear to be for only a year — and the experience they provide. The U.S. Department of Justice’s website in May included postings for “uncompensated” special assistant U.S. attorneys in California, Oklahoma, Maryland and West Virginia. Oklahoma’s posting was for two years, while the others asked for commitments of at most a year.

“My view is it could be criticized if special assistant U.S. attorneys were permitted to remain indefinitely unpaid employees,” said Rod Rosenstein, the U.S. attorney in Maryland. “We view it as sort of an internship.”

Rosenstein said his office has had 10 unpaid special assistant U.S. attorneys since 2010.

One of them, Kiran Patel, said the experience set him up well for his current law firm job.

“The (attorneys) who organized the program went out of their way to make sure that I got the opportunity to work on cases that aligned well with my interests and goals in terms of experience and skill development,” he said.

Justin Lee, who worked as an uncompensated federal prosecutor in Sacramento three years after graduating with a law degree from Georgetown University in 2008, handled misdemeanor cases such as traffic violations on federal land. The office hired him for a paid position right after his unpaid stint, but he said the vast majority of the 20 or so unpaid prosecutors who have come through Sacramento since 2010 didn’t land such full-time jobs with the office.

“When I looked at the positon being unpaid, that gave me pause,” he said. “But the potential benefit far outweighed that.”

The DOJ did not respond to repeated requests for comment about the jobs.

In a letter to the agency in 2012, the National Association of Assistant United States Attorneys said the unpaid positions were illegal under a law that forbids government agencies from receiving volunteer services for positions that have salaries fixed by Congress and require the same work as paid employees. Congress enacted the law in part to force agencies to operate within their budgets, the letter said.

Former unpaid special assistant U.S. attorneys said their work was similar to those in paid positions and included interviewing witnesses, writing briefs and appearing in court. Base salaries for assistant U.S. attorneys currently start around $52,000.

In a letter responding to the association, the director for the Executive Office for U.S. Attorneys, H. Marshall Jarrett, said Congress gave the attorney general authority to appoint special assistant U.S. attorneys and did not fix a minimum salary for those positions. That allows the department to appoint special assistant U.S. attorneys at a “gratuitous rate of pay,” he wrote.

Legal arguments aside, critics say the DOJ is limiting the pool of applicants for these positions to people who can afford to take a year or more off without pay — cutting out those from disadvantaged backgrounds and reducing the diversity of its prosecutors.

“Especially when we’re dealing with the administration of justice and making sure that it’s fair, having people of diverse backgrounds is really important,” said Rasha Gerges Shields, former deputy chief of the organized crime and drug enforcement task force at the U.S. Attorney’s Office in Los Angeles. Shields is now a partner at the law firm of Jones Day, where she heads the firm’s diversity committee in Los Angeles.

Lee said he understood the diversity concerns. He took out loans for school and would not have been able to take an unpaid year of work without the financial support of his family, he said.

But Rosenstein said his office is clear that uncompensated special assistant U.S. attorneys do not have an advantage over other applicants for permanent positions.

“They are not displacing anybody else,” he said. “We treat everybody equally.”