Just eight months ago, former federal prosecutor Renato Mariotti landed a first-of-its-kind conviction for financial “spoofing.”

But a year before that, Mariotti was already trying to figure out how he would explain “spoofing” to a jury. He decided to simplify things — and take jurors back in time to their playgrounds at school.

“There was a kid who would put his hand out like this, like he was trying to shake your hand,” Mariotti said during closing argument. “And he’d pull it away right when you were about to shake his hand.”

The analogy worked. It would take jurors barely an hour that day to hand Michael Coscia the dubious distinction of becoming the first person ever found guilty of spoofing, or illegally disrupting the financial markets with fraudulent orders that existed only for hundreds of milliseconds — like the kid who puts out his hand just to take it away.

The jury also handed Mariotti a landmark conviction in what would turn out to be the waning days of his nine-year career at the U.S. Attorney’s Office in Chicago. Mariotti joined Thompson Coburn as a partner in the firm’s business litigation practice in Chicago in late June. He is expected to focus on representing individuals and corporations in complex criminal matters, internal investigations and high-stakes litigation.

Mariotti leaves behind what he describes as a “very, very fun job” as a prosecutor. He convicted and landed a 36-year prison sentence for Joseph “Jose” Banks, one of the two bank robbers who notoriously escaped from jail in 2012 by rappelling down the side of the Metropolitan Correctional Center with a rope made out of bed sheets and dental floss. Mariotti also led the fraud prosecution of Block 37 developer Laurance Freed. And he quarterbacked the notorious case against the Bogdanov Family, a trio of traveling toy thieves.

Coscia still faces sentencing later this month. But sentencing hearings can be delayed. And with a handful of significant trials behind him, Mariotti said it was time to switch gears. As much as he enjoyed being a prosecutor, he said he “couldn’t do it forever.” He just wanted to feel like he did something “really meaningful” during his time in public service.

Looking back, Mariotti said, “I felt like I did.”

This is the wanted poster for Jose Banks, left, and, Kenneth Conley, who escaped from the Metropolitan Correctional Center on Tuesday, Dec. 18, 2012. Former prosecutor Renato Mariotti led the case against Banks. | Associated Press

This is the wanted poster for Jose Banks, left, and, Kenneth Conley, who escaped from the Metropolitan Correctional Center on Tuesday, Dec. 18, 2012. Former prosecutor Renato Mariotti led the case against Banks. | Associated Press