By Steve Gorman LOS ANGELES (Reuters) - Eighteen current or former members of the Los Angeles County Sheriff's Department have been charged with corruption and civil rights violations in a probe of inmate abuse inside the United States' largest county jail system, federal prosecutors said on Monday. Four indictments and a criminal complaint unsealed on Monday include accusations that sheriff's deputies subjected inmates and visitors at two downtown Los Angeles lockups to unjustified beatings or detentions and tried to cover up their wrongdoing. The highest-ranking officials charged in the probe were two sheriff's lieutenants - one who oversaw the department's Operation Safe Jails Program and another who was assigned to the internal criminal investigations bureau, prosecutors said. The pair were among seven accused in one indictment of conspiring to obstruct a 2011 federal investigation into allegations of excessive force and the smuggling of contraband by jail deputies in exchange for bribes. The indictment says the two lieutenants and others went so far as to try to prevent contact between federal investigators and an inmate informant after his cover was blown, altering records to make it appear the informant had been released from jail, then re-booking him under false names. Two separate indictments charged several sheriff's deputies with various civil rights violations, accusing some of using unjustified force against inmates, then trying to cover up the abuse, and others with the wrongful detention of various jail visitors, including an Austrian diplomat and her husband. Additionally, two other cases described by federal prosecutors as "spinoff" investigations led to mortgage-fraud charges against three other deputies - all brothers. A fourth deputy was indicted separately on weapons charges. U.S. Attorney Andre Birotte Jr told a news conference that 16 of the defendants were arrested on Monday and two others were expected to turn themselves in shortly. He said the investigation was continuing. Sheriff Lee Baca, whose 10,000-member department oversees the county jail system, said his agency cooperated with the FBI in its probe, adding that "while the indictments were not unexpected, it is nonetheless a sad day for this department." "We do not tolerate misconduct by any deputies," Baca said. This department is grounded in its core values, namely to perform our duties with respect for the dignity of all people and the integrity to do what is right and fight wrongs. The arrests come more than a year after a blue-ribbon commission blamed Baca for failing to halt what the panel called a persistent pattern of excessive force against inmates by his deputies, dating back years. In a statement announcing the criminal charges, Birotte said his investigation found that the alleged abuses "did not take place in a vacuum - in fact they demonstrated behavior that had become institutionalized." But Birotte declined to comment when asked whether federal authorities had any evidence that Baca or others in the upper echelons of his department were aware of the misconduct charged in the indictments. "I'm not here to discuss anything other than the charges here today," he said. The Los Angeles County jail system ranks as the largest in the nation, housing some 18,000 inmates. Reporting by Steve Gorman; Editing by Cynthia Johnston, Diane Craft and Jackie Frank