If you’re ever unfortunate enough to find yourself accused of assaulting a law enforcement officer, good luck. When it comes down to your word against the officer’s, and there are no impartial witnesses, you may end up in a jumpsuit even if you’re innocent.

But if you’re an inmate accused of assaulting a jailer, you’re in a considerably worse jam. And in the opinion of the ACLU of Southern California, your best chance of avoiding prosecution may be evidence that is routinely concealed by the L.A. County Sheriff’s Department and the district attorney’s office.

What evidence?

Past complaints of brutality by the very deputies you’re accused of assaulting. Such information is often unknown to the judge, the jury, the defendant or his lawyer. And, as one defense lawyer put it, you’re at a huge disadvantage because “you don’t know what you don’t know.”

On Tuesday morning, the ACLU’s chief local counsel, Mark Rosenbaum, announced the filing of a civil rights lawsuit against Dist. Atty. Steve Cooley and Sheriff Lee Baca, alleging that they’ve deliberately concealed evidence of deputy assaults from defense attorneys. Outside lawyers working with the ACLU on the case include law professors from USC and Harvard.

And the official response from downtown?

Cooley’s office called the lawsuit a “blatant attempt to mislead the public and the court,” and a Baca spokesman did not respond to my queries. One would hope that in the days to come, they’ll have a little more to say, because the allegation here is that they’ve corrupted the system by denying some defendants the chance for a fair trial.

Look, the L.A. County inmate population includes a lot of hard-core thugs, gangsters and knuckleheads, and being a guard in a dungeon like Men’s Central Jail is no picnic. But if you’ve been following The Times’ coverage the last 18 months or so, you know that some of the thugs are wearing badges.

Just last week, a captain in the Sheriff’s Department testified about a culture of brutality in which excessive force was tolerated and protected, with one supervisor allegedly warning deputies that when it was time for a tuneup, they should avoid hitting inmates in the face because the damage would be too obvious. And, as reported previously, jailhouse beatings have been witnessed by clergy and civilian jail monitors.

All of which is scandalous enough. But on Tuesday, Rosenbaum blasted what he called “blatantly illegal policies” of withholding information that could be of benefit to accused inmates, and he said requests for such information often go nowhere because “that information has been buried.”

“It mocks our entire system of fair trial,” Rosenbaum said.

The courts will eventually sort out whether Rosenbaum’s allegations have merit. But in the meantime, the ACLU presented a compelling witness. Jonathan Goodwin, a former inmate, appeared at the news conference to say he was assaulted at Men’s Central Jail in 2010 by six or seven deputies, only to find himself on trial for assault against the very deputies who jumped him.

Goodwin, who was locked up for violating parole after serving three years for robbery, described a minor disagreement with a guard in December 2010 over the way his food tray was shoved into his cell. Goodwin said he was later ordered out of his cell, pepper-sprayed, jumped by “six or seven” deputies, punched and kicked.

He said he did not resist, much less land any blows, and yet, five weeks later, he was charged by the district attorney with assault.

Goodwin said his lawyer tried to get information about prior complaints against the deputies involved, but he struck out. Only after a call by his attorney to the ACLU did his attorney discover that there had been allegations of similar incidents involving some of the same deputies.

Based in part on that information, Goodwin — who was told he could serve 17 years if convicted — was acquitted of the charges against him.

“I am lucky to be here rather than in state prison, but I am sure that there are lots of other people who are not so lucky,” Goodwin said Tuesday.

Meanwhile, attorney Tracy Grayson told me by phone that he represents a client, Arthur Townsend, who stands accused of assault against jailhouse guards.

“We really didn’t get anything,” he said of his attempts to obtain information about whether there were prior complaints about those deputies. Only after contacting the ACLU, which has been compiling information on accused officers, did Grayson get evidence that might help his client’s cause at trial.

“In my opinion, they’re playing a lot of games,” Grayson said of sheriff’s officials, telling me he is often kept waiting for two or three hours when he tries to visit his client in jail.

Benjamin Gluck, a lawyer with Bird, Marella, worked on the ACLU-supported lawsuit. He said Tuesday that “for any justice system to merit the name, those who enforce the law must themselves obey it.” But Cooley, he said, was disregarding a legal obligation to share information — known as exculpatory evidence — that can benefit the defense or harm the prosecution in criminal cases.

Gluck said Cooley’s policy, clearly stated on the D.A.'s website, gives prosecutors broad discretion on whether or not to share certain information. Other jurisdictions, Gluck said, including the Ventura County district attorney’s office, have rejected L.A. County’s interpretation of the law and have adopted standards that would potentially allow for more information to be released to the defense.

Still unknown is how many inmates stand wrongly accused, or how many took pleas and have served time because they saw no chance of proving their innocence.

As you may know, Jackie Lacey and Alan Jackson are in a runoff to succeed Cooley. If they’d like to weigh in on the issue, they know where to find me.

steve.lopez@latimes.com