"ER" producer John Wells says he is still searching for the ideal way to conclude his U.S. TV series after 15 seasons.

Wells said while the overall finale concept for the long-running medical drama has been determined, he continues to work on the details that will offer "ER" fans a satisfying conclusion, the Los Angeles Times reported Sunday.

"You want to try and find the essence of the series," he said "You want to find the thing that people actually identify with in the series and do something that leaves them feeling satisfied for having spent X number of hours of their lives devoted to watching your ongoing narrative."

The Times said one key issue facing the producer, who has worked on series such as "China Beach" and "The West Wing," is how to reintroduce actor George Clooney to the NBC series.

Clooney is among several former "ER" stars who have agreed to appear during the series' final season, which is to end March 12.