Apple Chief Executive Steve Jobs showed up for work on Monday and was spotted leaving the main corporate campus in Cupertino, California, a Reuters witness said.

Speculation had mounted in past days that Jobs had returned to the company he co-founded more than 30 years ago, and emerged after a nearly six-month medical leave.

On Monday, he was seen leaving the main Apple building in Cupertino, California and getting into a black car alone that was then driven off by men in black suits with ear-pieces.

The Wall Street Journal reported that Jobs, 54, underwent a liver transplant in Tennessee while on leave. The company has declined to comment on the report.

Jobs is viewed as the key visionary driving the company's product development and the mastermind behind iconic products such as the iPod and the iPhone.

(Reporting by Clare Baldwin; Editing by Richard Chang)

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