UPDATE 1-Sunoco says Marcus Hook FCC at planned rates

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HOUSTON, June 3 (Reuters) - Sunoco Inc (SUN.N) said a gasoline-producing fluidic catalytic cracking unit shut by a May 17 explosion at its 178,000-barrel-per-day (bpd) Marcus Hook, Pennsylvania, refinery was back in operation at planned rates on Wednesday.

Traders and state regulators had said Sunoco was planning to restart the FCC this week.

The unit was shut by an explosion in a nearby ethylene unit that was felt miles away. The cause of the explosion remains under investigation, a Sunoco spokesman said.

"We are committed to completing a thorough investigation," said Sunoco spokesman Thomas Golembeski. "No one wants to know the cause more than we do."

New York Mercantile Exchange gasoline and heating oil futures spiked on news of the blast. (Reporting by Erwin Seba; Editing by David Gregorio)