Former Representative Jon Christensen, a Nebraska Republican, filed a termination report to the U.S. Senate's lobbying disclosure database saying he stopped representing the company as of July 13, a day after news broke of the FBI investigation.

Christensen, who served in Congress in the 1990s, did not respond to phone calls or emails on Thursday.

"ZTE doesn't comment on personnel matters," said Anna Hughes of Ogilvy Public Relations, speaking on behalf of ZTE.

Christensen's departure was first reported by Politico.

The FBI probe and a separate one by the U.S. Department of Commerce were triggered by a Reuters report in March that Shenzhen, China-based ZTE had sold Iran's largest telecom a surveillance system capable of monitoring landline, mobile and Internet communications.

A day after the Reuters report, the Commerce Department issued a subpoena to ZTE. The company's general counsel in Texas, Ashley Kyle Yablon, told FBI agents that ZTE officials at that point discussed shredding documents relevant to the subpoena, according an FBI affidavit first reported by the website The Smoking Gun.

Yablon told the FBI the company used "sub companies" to buy sophisticated U.S. telecommunications equipment to get around restrictions on selling to countries like Iran.

ZTE, which sells equipment in 140 countries, according to its website, reported revenues in 2010 of $10.6 billion.

Primarily known for its smart phones, ZTE has a subsidiary specializing in surveillance and security technology.

The United States first imposed trade sanctions on Iran in 1979. More recently, the United States has joined other nations in additional trade sanctions based on its alleged nuclear weapons program.

(Reporting by Marcus Stern; Editing by Marilyn W. Thompson and Jim Loney)

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