The undisclosed role of informants in several high-profile cases has roiled the county's criminal justice system and raised questions about the conduct of prosecutors and sheriff's jailers.
Earlier this year, a panel established by Dist. Atty. Tony Rackauckas released a highly critical report saying a "failure of leadership" in his office led to the scandal.
Justice officials announced the investigation Thursday, saying the inquiry would scrutinize whether changes are needed to the long-running practice of deploying informants to surreptitiously extract information from defendants.
Investigators will search for patterns of widespread violations of the Constitution's 6th Amendment — namely whether defendants were denied their right to have their attorney present during any questioning. They will look as well at whether prosecutors have adhered to strict rules that require them to disclose evidence that is favorable to defendants.
"A systematic failure to protect the right to counsel and to a fair trial makes criminal proceedings fundamentally unfair and diminishes the public's faith in the integrity of the justice system," Vanita Gupta, head of the Justice Department's civil rights division, said in a statement.
If problems are found, federal authorities could present Rackauckas with a set of reforms and offer him a stark choice: Make the changes or face a federal lawsuit.
In announcing the investigation, Gupta and U.S. Atty. Eileen Decker made clear they expected Rackauckas to cooperate, highlighting an offer he made this year for federal officials to look into his office's use of informants.
"We appreciate the district attorney's invitation to review his office's policies and practices, along with his assurance of unfettered access to documents and personnel in his office," Decker said.
Rackauckas and Sheriff Sandra Hutchens, whose jailers run the informant program, said they would cooperate with the probe.
Rackauckas said he would "provide all needed documents and information." He added that he expects Justice officials will find that his department "did not engage in systematic or intentional violation of civil rights of any inmate and no innocent person was wrongfully convicted."
Some reliance on informants is legal as long as they are not part of an orchestrated plan to elicit information from a suspect. When informants are actively drafted by police and prosecutors, they effectively become an arm of law enforcement. In such instances, their involvement can violate a defendant's right to counsel under the U.S. Constitution.
However, police and prosecutors often deny overstepping legal bounds.
Daniel Medwed, a professor of law and criminal justice at Northeastern University and an expert on wrongful convictions, said there is a belief among defense attorneys that the use of informants is often based on a "wink and a nod" instead of an explicit arrangement.
The other problem prosecutors face when relying on jailhouse informants is disclosure. If the informant is offered a benefit in exchange for information — such as a shorter jail sentence — the defense must be informed. Failure to do so could violate a defendant's right to a fair trial.
In the past, both Rackauckas and Hutchens have rejected claims that the use of informants has led to anything other than isolated problems.
The decision by federal officials to delve into the inner workings of the county's top law enforcement agency raises the stakes dramatically in what has been a rancorous battle over the use of jailhouse informants.
Last year, a judge kicked Rackauckas' office off the case of Scott Dekraai, who pleaded guilty to killing eight people at a Seal Beach salon in 2011.
The district attorney's office was pursuing the death penalty, but Superior Court Judge Thomas Goethals handed the case over to the state's attorney general. Goethals made the dramatic move after growing increasingly impatient with sheriff's jailers and prosecutors, who resisted repeated orders to fully disclose the role of an informant in the case and to hand over records about the informant program. An appeals court recently upheld the decision, saying there was ample evidence of wrongdoing.
The discoveries in the Dekraai case have upended several other murder cases. In February, a Superior Court judge ordered a new trial for a convicted killer on the grounds that authorities failed to reveal that a key witness was an informant.
In another case, prosecutors neglected to tell defense lawyers they had used a paid jailhouse informant to help convict a man of a 2004 gang slaying in Santa Ana. In light of that information, the murder conviction was scuttled and the man was given a 15-year sentence for manslaughter. He could be out of prison in four years.
And a defendant who faced the possibility of life in prison if convicted of murder in two gang killings went free on probation after being allowed to plead guilty to second-degree murder. Prosecutors lost faith in the case after hearings in the Dekraai case revealed that an informant gathered information on the defendant.
Medwed said the federal investigation into Orange County informants "has been a long time coming."
While first used to infiltrate complex organizations such as corrupt governments and the mafia, the use of informants has expanded over time to include murder and other crimes, Medwed said.
Prosecutors, he said, have come to over-rely on informants, especially in weaker cases. Informants factor into a high percentage of cases in which defendants are later found to have been wrongfully convicted, he said.
"The problem with informants is they're so attractive. If it's a weak case, maybe bringing in an informant is a way to solidify it," Medwed said.
The government's move Thursday was foreshadowed by the committee Rackauckas appointed as he tried to address the blossoming crisis. The panel, which included legal experts, concluded that the district attorney's office was functioning "as a ship without a rudder" and faulted some of its prosecutors for adopting a "win-at-all-costs mentality."
Specifically, it suggested a series of reforms, including the need to create "a clear and consistent policy regarding its use of jailhouse informants" that would establish strict limits on when and how they are used.
After the panel presented its findings to Rackauckas, the district attorney wrote to U.S. Atty. Gen. Loretta Lynch in January to request the federal inquiry.
If federal officials find that jailers and prosecutors routinely violated defendants' rights with the use of jailhouse informants, they are likely to impose a consent decree on the county that would require changes and appoint a monitor to track progress.
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3:40 p.m.: This article was updated with additional details and reaction.
1:40 p.m.: This article was updated with comments from federal and local justice officials.