LOS ANGELES — Any foreign movie knocking on China’s door must pass through powerful gatekeepers — the China Film Group and its chief executive, Han Sanping.


The China Film Group functions as the Chinese government’s guardian of a film market that recently shot past Japan’s to become the world’s second-largest in box-office receipts behind the United States. On a broad array of business dealings — censorship, distribution and co-productions, among others — it is the conduit for foreign moviemakers hoping to make or distribute films in China.


But Mr. Han and his group are also supervising a trade route that is suddenly under close watch by regulators in Washington, after reports last week that officials in the United States are examining whether American film companies have violated domestic law by making illegal payments to officials in China.


In March, the Securities and Exchange Commission wrote to major film companies and smaller competitors — including Walt Disney, 20th Century Fox and DreamWorks Animation — requesting information about their business practices in China, according to people with knowledge of the investigation, who spoke on condition of anonymity because the matter could end up in court.


The investigation was set off by a whistle-blower, one of the people said. It is not known specifically which American business dealings with China are under investigation, but this person said the Hollywood companies were told to freeze all files, e-mails and other data related to getting films made or distributed in China. Several people briefed on the letters described some aspects of them, but all spoke on condition of anonymity because of confidentiality strictures.


While the letters are said to include no specific reference to China Film, executives scrambling to comply with the request are preparing as if their dealings with the group are part of the inquiry because of its all-important role in getting films made in China, according to executives based in the United States and in China.


The S.E.C. inquiry, which would have direct legal consequences only for companies with an American presence, is certain to make Hollywood studios that do business in China even more wary of possible missteps.


It has also focused fresh attention on the official Chinese apparatus for making and distributing films, beginning with the State Administration of Radio Film and Television and extending to the China Film Group, a state-controlled entity without whose support a foreign film is not allowed to enter China.


“It’s the equivalent of Universal, Sony, the M.P.A.A. and Regal all tied up in one,” said an American producer who has done business extensively in China. His description compared the China Film Group to a pair of major studios, the Motion Picture Association of America trade group, and America’s largest theater chain, Regal Entertainment Group, but with the added authority of a government franchise.


In China film circles, Mr. Han’s name requires little explanation: He is called “Master Han” or “the godfather of the Chinese movie industry.”


He had a role in directing “The Founding of a Republic,” and “Beginning of the Great Revival,” a pair of patriotic Chinese epics about Mao and the founding of the Communist Party.


That role comes atop producing credits on more than four dozen movies, most of them Chinese, but at least two — “The Karate Kid,” from Sony Pictures Entertainment, and “Mission: Impossible III” — that were made by Hollywood studios for global markets with involvement by the China Film Group.


In person, Mr. Han, who speaks little or no English, can nevertheless come across as an almost stereotypical Hollywood producer, an acquaintance said. He is demonstrative and colorfully expressive when speaking, and appears eager to befriend and be seen with Western movie stars, the acquaintance said.