(Recasts with investigators confirming bomb was involved)

LITTLE ROCK, Feb 4 (Reuters) - A bomb caused the explosion that wounded the chairman of the Arkansas State Medical Board in his car on Wednesday, federal investigators said.

The Bureau of Alcohol, Tobacco, Firearms and Explosives "has confirmed that it was caused by an explosive device," spokesman Drew Wade said.

He said five agents were on the scene in the town of West Memphis and that the federal law enforcement agency was leading the investigation of the attack on Dr. Trent Pierce.

Earlier, there was speculation the blast may have had a mechanical cause. The car was reported to be a Lexus hybrid.

"We do know that he was very, very seriously injured," Bill Trice, the attorney of the state's medical board, told Reuters. "Our board members are in shock and waiting to hear any news, and are very saddened and prayerful for him and his family." (Additional reporting by James Vicini in Washington) (Reporting by Steve Barnes and Ed Stoddard; Editing by John O'Callaghan)

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