The US' House Foreign Affairs Committee said last week it will investigate Yahoo! over allegations that it covered up its involvement in handing over information to Chinese authorities to help them pursue political opponents.
Yahoo! has always said its Chinese subsidiary was only obeying local laws.
Tom Lantos, chairman of the committee, said: "It is bad enough that a wealthy American company would willingly supply Chinese police the means to hunt a man down for shedding light on repression in China. Covering up such a despicable practice when Congress seeks an explanation is a serious offense. For a firm engaged in the information industry, Yahoo! sure has a lot of secrecy to answer for. We expect to learn the truth, and to hold the company to account."
Yahoo! handed over information about Shi Tao, a Chinese journalist. He was sentenced to 10 years' imprisonment for posting on a website called Democracy Forum.
But the company always said it did not know what he was being investigated for. Documents apparently sent to Yahoo! make clear this was untrue.
Documents from Beijing State Security Bureau said they were investigating "illegal provision of state secrets to foreign entities" - commonly used to capture political dissidents.
The request asked Yahoo! for email details, log in times, IP addresses, and email content from February 2004 until 22 April 2004 when the letter was sent.
Shi is appealing his sentence in China and is also seeking damages against Yahoo! in the US courts.
The committee's statement is here.