The U.S. special operations forces captured the operative more than a week ago, if not longer ago than that, a U.S. official told ABC News today.
Two U.S. officials describe the ISIS figure as being a mid-level operative who is being interrogated by the U.S. military but will eventually be turned over to Iraqi forces.
Pentagon officials declined to comment on the record today about the seizure of the ISIS operative.
The Expeditionary Targeting Force arrived in Iraq earlier this year and since then has been developing an intelligence and source network to be used for potential ISIS targets, U.S. officials said.
Carter told reporters on Monday that the ETF was "in position" in Iraq and "it is having an effect and operating." Though he did not provide specifics of the unit's operations, Carter said he expected it would "be a very effective part of our acceleration campaign. I don't have any more on that."
Currently, the ETF numbers close to 200 personnel, including support staff, and is being staffed by members of the elite Delta Force, according to U.S. officials.
When operating inside Iraq, the ETF's operations will be coordinated with Iraqi authorities, however, they will act unilaterally in Syria if needed, officials said.
Peter Cook, the Pentagon press secretary, declined to confirm the detainee's capture for operational security reasons.
"As the Secretary said on Monday, the ETF has begun operations in Iraq, but we will not discuss the details of those missions when it risks compromising operational security," Cook said. "One of the goals of the ETF is to capture ISIL leaders. Any detention would be short-term and coordinated with Iraqi authorities."
ISIS is also known as ISIL or the Islamic State.