WASHINGTON (Reuters) - A former Connecticut resident and Chinese citizen who allegedly worked on the F135 engine was charged with trying to take sensitive documents about military technology to China, according to a criminal complaint unsealed on Tuesday.

Yu Long was en route to China in November via Newark Liberty International Airport in New Jersey where customs officers found proprietary documents in his luggage that included equations and test results used to develop titanium for U.S. military aircraft, prosecutors said.

Yu Long was arrested two days later in Ithaca, New York.

U.S. defense officials have sounded the alarm in recent years about increased efforts by China, Russia and other countries to gain access to U.S. military technologies.

Between 2008 and earlier this year, Yu Long worked as a senior engineer for a Connecticut defense contractor. He had said he worked on the F119 and F135 engines, which are used in U.S. fighter aircraft and manufactured by United Technologies Corp.

Yu Long worked for the United Technologies Research Center, which does research for the conglomerate, a source familiar with the case said. He was let go in May when the probe began, said the source, who was not authorized to speak publicly.

The company said in a statement it was cooperating with authorities, but had no further comment since the investigation was ongoing. A lawyer for Yu Long did not immediately respond to a request for comment.

Prosecutors said Yu Long was also stopped at John F. Kennedy International Airport in August and found to have $10,000 in undeclared cash, registration documents for a new Chinese corporation and an application to work for a state-controlled aviation research center in China that highlighted Yu Long's work experience on the engines.

The United States has long prohibited the export of U.S. defense technology and information to China.

(Reporting by Aruna Viswanatha and Andrea Shalal; editing by G Crosse)