Long Hill Township Police Department David Bird

An unexpected clue has emerged in the case of a Wall Street Journal energy reporter who went out for a walk five days ago and never returned: His credit card was used in Mexico on Wednesday night, a source close to his family told NBC News.

Hundreds of people have been searching for David Bird since he vanished in New Jersey on Saturday after he stepped out of his house without his cellphone or the medicine he takes twice daily for his liver transplant.

His family does not believe he simply wandered away and has suspected that his disappearance might be rooted in his work, which includes coverage of OPEC, a collection of oil-rich countries.

Long Hill, N.J., Police Lt. Ahmed Naga said police were trying to confirm the lead.

Naga said he was with a team that was using cameras to search under the ice on the Passaic River.

"It's a big, big activity," he said.

The Federal Bureau of Investigation is also assisting officials.

”The FBI New Jersey office is assisting the Long Hill Township police and the Morris County Prosecutor's office in this investigation,” said a spokesman for the FBI in New Jersey.

Bird, 55, was putting away Christmas decorations with his wife, Nancy, at their Long Hill Township home when he decided to take a stroll before a rainstorm, his wife told the Journal.

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When he had not returned in a couple of hours, even though it was raining heavily, his wife called police, who began searching the woods near the house.

"I knew this wasn't right. Something was wrong," the wife said. "He's a really great dad and a really great husband."

She said her husband, who was recovering from a stomach virus, could fall ill without the medication he has taken since a 2004 liver transplant.

Her husband, a father of two boys, is an experienced hiker and camper who ran the 2013 New York City Marathon.

"This is so unlike him it's unbelievable to me," fellow Boy Scout leader Jim Caparoso told the newspaper. "If he was stuck somewhere, he knows how to shelter himself enough out of the elements. He knows how to signal if he can."

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