Jarek Koska is the head of an Eastern-European gang named SP-9, which runs a kidnapping ring and is also responsible for huge money laundering operations. Even though SP-9 is street gang, under Koska it became a highly organised and very dangerous force.
Koska and SP-9 laundered millions of dollars through Onestate Bank with the help of Angela Markham who deactivated the software capable of detecting money laundering transactions. So when Angela Markham was arrested for a hit-and-run case, Koska had his men kidnapped Samuel Gates, Jr. to force Judge Samuel Gates to release Markham. In the process his men tortured and killed Samuel Gates Jr's nanny Christina Rojas and injured Reese.
After successfully getting Angela Markham released Koska planned on killing both Judge Samuel Gates and his son. However, Reese kidnapped Angela and had Finch transfer all of Koska's money to an off-shore account. Reese threatened that if Judge Gates and his son weren't released, Koska wouldn't get his money back and his own clients would kill him. Koska ordered his men to kill everyone as their operation was "burned", but Reese non-fatally wounded all of Koska's men before shooting Koska himself in the leg and the arm. Jarek Koska, Angela Markham and the rest of his men were left tied up for Carter and Fusco and the police to find along with the laundered money.