Cook County prosecutors have been ordered to drop all cases handled by nine special operations Chicago police officers, four of whom have been charged with robbing, kidnapping and intimidating drug dealers, according to a memo sent to state's attorney's offices this month.

The order applies to at least 110 arrests made by the officers for crimes including weapons violations, drug possession and battery, according to the Sept. 12 memo acquired by the Chicago Tribune and detailed for Thursday's editions.

The four officers — Keith Herrera, 28; Jerome Finnigan, 43; Thomas Sherry, 32; and Carl Suchocki, 32, all of Chicago — face charges that include multiple counts of armed violence, home invasion, aggravated kidnapping and delivery of a controlled substance.

Prosecutors decided to drop the cases connected to the four charged officers and to five others who are under investigation and have been stripped of their police powers but not charged.

Bernard Murray, chief of felony prosecutions for the state's attorneys office, said prosecutors are "trying to do the right thing."

"We made the decision to err on the side of caution," he said.

The memo directs prosecutors to drop cases that may have been tainted by the officers' involvement, including those in which the officers made the arrest, signed a search warrant, gave information from an informant or recovered physical evidence.

"We cannot prove our cases," Murray said. "The cases where the defendant was indicted and they were the main officers, we cannot sustain the burden of proof."

Officials say they believe some of the arrests weren't made legitimately and that all of the cases are now thrown into question.

Earlier this month, prosecutors dropped at least 27 "tainted" drug and gun cases after the officers were arrested, including a case against two men who were nabbed last fall when police seized $15 million worth of cocaine, officials said.

All four officers have been released from jail after a judge ruled that their bail money came from legitimate sources. They face up to 30 years in prison if convicted.

All have been suspended without pay, and police officials say they intend to fire them.