A Chinese professor who faced charges of espionage for selling scientific secrets to China that were later dropped has filed a lawsuit claiming that the lead FBI agent in his case falsified evidence and ignored warnings that he was innocent.

Xi Xiaoxing, a naturalized US citizen from China who teaches physics at Temple University in Philadelphia, filed the lawsuit there on Wednesday, according to The New York Times. The suit seeks an unspecified amount of damages against the agent.

In May 2015, FBI agents arrested Xi at his house at dawn, handcuffed and held his wife and young daughters at gunpoint, according to the lawsuit, the newspaper reported on Thursday.

Xi's lawyer, David Rudovsky, told the Times, that the FBI had filed and dropped two other cases against Chinese-American. He said that he wants to know if the FBI is profiling Asian-American scientists in its efforts to fight spying. He said there was nothing secret about the material Xi sent to academic colleagues in China.

"It is troubling that several cases against Chinese Americans have been dismissed where it appears that there was no evidence of guilt," Rudovsky wrote in an email to China Daily.

Peter Zeidenberg, who is with the Washington-based firm of Arent Fox LLP, was Xi's attorney in 2015 when the world-renowned researcher on superconductivity was accused of passing the schematics of a pocket heater used in semiconductor research to China. After he pleaded not guilty, the charges were dropped several months later.

"We investigated and got affidavits from notable physicists from around the world," Zeidenberg recalled in an interview with China Daily on Thursday. "We presented the evidence to the government and they agreed to dismiss the case."

Zeidenberg, who is not representing Xi in his lawsuit against the FBI agent (Andrew Haugen), said he believes the US government is targeting Chinese when investigating alleged espionage cases. He said that he has been involved in other cases where the US government pursued Chinese, "maybe 10 in the last two years".

"The DOJ (Department of Justice) has a laser-like focus on all things related to China. When you put on the blinders sometimes you miss warning signs," he said. "I don't think they (federal government) have learned a lesson. I expect the trend (of focusing on the Chinese) to continue."

Zeidenberg represented Xiafen "Sherry" Chen, also a naturalized citizen, who worked for the National Weather Service. She was accused of using a stolen password to download information on US dams and reservoirs from a secure database, which coincided with a meeting with a top Chinese official in 2014.

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