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By The Associated Press

BEIJING — Chinese authorities are investigating a company that researchers say sold North Korea materials that can be used by its growing nuclear weapons program.

The announcement about Hongxiang Industrial Development Co. in Dandong, a northeastern border city in Liaoning province, was unusually explicit for Beijing, whose dealings with the North are shrouded in secrecy.

The crackdown reflects Beijing's growing frustration with its isolated neighbor.

Hongxiang is suspected of unspecified "serious economic crimes," according to separate announcements by police in Liaoning and China's foreign ministry.

Related: U.S. 'Should Be Increasingly Worried' by Kim's Nuke Tests

They gave no details, but a South Korean think tank said last month the firm supplied aluminum oxide and other materials that are used in processing nuclear bomb fuel.

According to the Asan Institute for Policy Studies, Hongxiang supplied the North with possible "dual use" products including pure aluminum ingots, aluminum oxide, ammonium paratungstate and tungsten trioxide.

Tungsten trioxide can be used in making more aerodynamically stable missiles, while aluminum oxide is used to resist corrosion in gas centrifuges during uranium enrichment, according to Asan. It said those could qualify as "potential military and nuclear dual use products" under U.S. Department of Commerce restrictions.

Related: Why Isn't China Reigning In Nuclear-Armed North Korea?

Hongxiang is one of the biggest traders with North Korea and carried out imports and exports worth a total of $532 million in 2011-15, according to the report.

Companies affiliated with it also helped to maintain North Korea's cyber infrastructure and traded with entities in Myanmar that also are subject to international sanctions, Asan said.

There was no indication why the large scale of Hongxiang's trade failed to attract official attention earlier.

Beijing has long been North Korea's main source of aid and diplomatic support but President Xi Jinping's government has shown growing frustration with Pyongyang's pursuit of nuclear weapons in defiance of foreign pressure.

The government of North Korean leader Kim Jong Un conducted its fifth nuclear test on Sept. 9. That raised concern abroad it was moving closer to its goal of a nuclear-armed missile that could one day strike the U.S. mainland.

Phone calls to Hongxiang's headquarters in Dandong on Wednesday were not answered. People who answered the phone at Dandong police headquarters said they had no information about the investigation. The Liaoning provincial police headquarters didn't respond to questions sent by fax.