Los Angeles County prosecutors and sheriff’s officials have for years concealed complaints about law enforcement misconduct and other important evidence from defendants in criminal cases, according to a lawsuit filed Tuesday by civil rights attorneys and legal scholars.

At a news conference announcing the suit, a lawyer with the American Civil Liberties Union of Southern California blasted the Sheriff’s Department and district attorney’s office for following policies he said played “fast and loose with evidence of innocence of those prosecuted.”

The lawsuit cited several cases in which authorities allegedly failed to disclose information about misconduct complaints filed by inmates against deputies who were to be witnesses in criminal cases. Attorneys behind the lawsuit claimed that similar evidence might have been kept hidden in far more cases — possibly thousands — over the last decade.

“In Los Angeles County, we have a system of injustice for all criminal defendants,” Mark Rosenbaum, chief counsel for the local ACLU, told reporters.

The Sheriff’s Department and district attorney’s office denied the allegations, saying the lawsuit mischaracterized how they decide what evidence is turned over.

“The lawsuit ... is a blatant attempt to mislead the public and the court,” Dist. Atty. Steve Cooley said in a statement. “This office is confident that our ... policy complies with the highest constitutional and statutory standards.”

In addition to filing a lawsuit, the ACLU submitted a state bar complaint against Cooley and called for a civil grand jury investigation as well as the appointment of an independent counsel to review cases that have resulted in convictions since the controversial policies were adopted.

The lawsuit’s claims go to the heart of the legal duty that prosecutors have to ensure defendants receive a fair trial by disclosing information favorable to the defense.

The broad outline of the district attorney’s policy about what should be disclosed dates to 2002 and has been hailed by some as a model. But others, including many defense attorneys, have criticized its approach.

Tuesday’s lawsuit contends that the district attorney’s office violates the rights of inmates by preventing prosecutors from disclosing information about law enforcement misconduct complaints and other evidence unless there is “clear and convincing evidence” that the information is true. That is a higher burden than the “preponderance of evidence” standard required for police departments to discipline or fire officers. Prosecutors in other counties, such as Ventura, do not require such a high standard, the ACLU said.

The suit also claims that the district attorney’s office improperly withholds evidence that involves ongoing investigations and requires prosecutors to decide for themselves what evidence would probably affect the outcome of the defendant’s case.

Harry Caldwell, a professor at Pepperdine School of Law, examined a copy of the district attorney’s policy at the request of The Times and suggested that prosecutors should be advised to more often ask judges whether certain evidence needs to be disclosed.

“Let the judge, that neutral, independent, detached magistrate, make the determination,” said Caldwell, a former Riverside and Santa Barbara county prosecutor who now represents inmates on death row.

The lawsuit also takes aim at the way the Sheriff’s Department keeps track of inmate complaints against deputies.

The suit cited testimony earlier this year by a sheriff’s lieutenant who acknowledged that the department does not keep inmate complaints in the personnel files of the deputies accused of misconduct, requiring officials to hand-search thousands of documents to find complaints against specific jailers.

Jonathan Goodwin was one of four inmates cited in the lawsuit who claim they were beaten by deputies but then falsely accused of being the aggressors and charged with assault. Goodwin’s attorney sought evidence of complaints against the deputies involved in the incident but was told none existed, the lawsuit said. Only when she contacted the ACLU did she discover that one of the deputies had been the subject of several excessive force complaints by inmates.

Goodwin was acquitted of assaulting deputies in May, according to the suit.

“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,” he told reporters.

Sheriff’s spokesman Steve Whitmore said the department has yet to review the lawsuit but disputed the claim that inmates are beaten and then falsely accused of assault. Assistant County Counsel Roger Granbo declined to discuss the lawsuit’s claims but said the Sheriff’s Department complied with a subpoena from a defense attorney earlier this year seeking inmate complaints against specific deputies.

“There has never been an attempt by anyone in the Sheriff’s Department to hide anything from anybody, especially the court,” Granbo said.

jack.leonard@latimes.com