Fresh slivered onions on the restaurant chain's Quarter Pounders "are the likely source" of the outbreak, the agency said Wednesday.

The number of E. coli cases linked to McDonald’s Quarter Pounders has risen to 90, the Centers for Disease Control and Prevention said Wednesday, up from 75 reported cases Friday.

The slivered onions used on the burgers are the “likely source” of the outbreak, the CDC said.

A spokesperson for McDonald’s said all Quarter Pounders in affected states had been pulled from the restaurants by Oct. 22, when the outbreak was announced.