The Alton Police Department improperly destroyed evidence from 130 pending criminal cases, according to Madison County State's Attorney Tom Gibbons.
It was not immediately known what kinds of evidence were destroyed or why.
Alton Mayor Brant Walker said he was "extremely angry and distressed" by the evidence destruction. He said the evidence officer, Jonathan Forrier, has been placed on administrative leave pending the investigation. Forrier could not be reached for comment.
"This was an egregious breach of protocol," Walker said, vowing a "thorough, transparent investigation" of the incident.
Alton Police Chief Jason Simmons issued a statement calling it "a very serious situation." He vowed transparency and continued cooperation with the investigation.
"The men and women of the Alton Police Department are some of the most professional and dedicated officers I have ever worked with," Simmons said. "I do not want this incident to overshadow the outstanding work of the Alton Police Department over the last year."
As required by law, Gibbons' office has filed notice in 130 pending criminal cases informing the defendants that the evidence against them has been destroyed.
"It is my constitutional and ethical obligation to ensure that justice and fairness are always given the highest priority and guarantee in our criminal justice system," Gibbons said. "In this matter, that begins by giving criminal defendants full and fair disclosure of the destruction of evidence in their cases."
Gibbons said he has enlisted the help of U.S. Attorney Steve Wigginton, the Illinois State Police and the FBI. He also is consulting with Walker and the Alton city attorney, whom he described as "fully cooperative and supportive of all our efforts."
A full audit is taking place of the entire evidence vault at the Alton Police Department, Gibbons said.
"We will take all necessary steps to ensure the integrity of all evidence that remains in the vault and to determine the cause of this serious situation," he said. "If the investigation reveals evidence of criminal wrongdoing, we will take appropriate action."
Walker said that the city also will undertake an internal investigation, though he does not believe there was a criminal intent to the evidence destruction.
"We'll have a better, clearer picture when all the investigations are complete," he said.
Simmons also said there would be an internal investigation, and a review of internal policies to ensure it doesn't happen again.
Despite the incident, Walker said he still has full faith and confidence in the Alton Police Department.
"This individual's actions do not reflect the entire Police Department," he said.