George Clooney has agreed to come back to the show that launched his career: ER.

Sources close to the series tell PEOPLE that ER co-creator John Wells has ordered a closed set to keep Clooney’s appearance under wraps.

It’s not known how many episodes will feature Clooney as the show films its final season. Neither NBC nor Warner Bros. reps would comment, nor did Clooney’s publicist.

Clooney, himself, denied reports last March he was returning.

Clooney’s Dr. Doug Ross was last seen on ER in a 2000 cameo walk-through in a remote lake scene with Ross’s ex-Juliana Marguilies.

Clooney is the most famous ER alum and Wells has always wanted him to return for the show’s last season. The 15th season was extended by an extra four episodes by NBC executives last week.

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