WASHINGTON >> When the Justice Department arrested the chairman of Temple University’s physics department this spring and accused him of sharing sensitive U.S.-made technology with China, prosecutors had what seemed like a damning piece of evidence: schematics of sophisticated laboratory equipment sent by the professor, Xi Xiaoxing, to scientists in China.

The schematics, prosecutors said, revealed the design of a device known as a pocket heater. The equipment is used in semiconductor research, and Xi had signed an agreement promising to keep its design a secret.

But months later, long after federal agents had led Xi away in handcuffs, independent experts discovered something wrong with the evidence at the heart of the Justice Department’s case: The blueprints were not for a pocket heater.

Faced with sworn statements from leading scientists, including an inventor of the pocket heater, the Justice Department on Friday afternoon dropped all charges against Xi, a U.S. citizen.

It was an embarrassing acknowledgment that prosecutors and FBI agents did not understand — and did not do enough to learn — the science at the heart of the case before bringing charges that jeopardized Xi’s career and left the impression that he was spying for China.

"I don’t expect them to understand everything I do," Xi, 57, said in a telephone interview. "But the fact that they don’t consult with experts and then charge me? Put my family through all this? Damage my reputation? They shouldn’t do this. This is not a joke. This is not a game."

The United States faces an onslaught from outside hackers and inside employees trying to steal government and corporate secrets. President Barack Obama’s strategy to combat it involves aggressive espionage investigations and prosecutions, as well as increased cyberdefenses.

But Xi’s case, coming on the heels of a similar case that was dismissed a few months ago in Ohio, raises questions about whether the Justice Department, in its rush to find Chinese spies, is ensnaring innocent U.S. citizens of Chinese ancestry.

A spokeswoman for Zane D. Memeger, the U.S. attorney in Philadelphia who brought the charges, did not elaborate on the decision to drop the case. In court documents, the Justice Department said "additional information came to the attention of the government."

The filing gives the government the right to file the charges again if it chooses. A spokesman for John P. Carlin, the assistant attorney general who is overseeing the crackdown on economic espionage, had no comment on whether Justice Department officials in Washington reviewed the case.

The science involved in Xi’s case is, by any measure, complicated. It involves the process of coating one substance with a very thin film of another. Xi’s lawyer, Peter Zeidenberg, said that despite the complexity, it appeared that the government never consulted experts before taking the case to a grand jury. As a result, prosecutors misconstrued the evidence, he said.

Zeidenberg, a lawyer for the firm Arent Fox, represented both Xi and Sherry Chen, a government hydrologist who was charged and later cleared in the Ohio case. A longtime federal prosecutor, Zeidenberg said he understood that agents felt intense pressure to crack down on Chinese espionage, but the authorities in these cases appeared to have been too quick to assume their suspicions were justified.

In Xi’s case, Zeidenberg said, the authorities saw emails to scientists in China and assumed the worst. But he said the emails represented the kind of international academic collaboration that governments and universities encourage. The technology discussed was not sensitive or restricted, he said.

"If he was Canadian-American or French-American, or he was from the U.K., would this have ever even got on the government’s radar? I don’t think so," Zeidenberg said.

The Justice Department sees a pernicious threat of economic espionage from China, and experts say the government in Beijing has an official policy encouraging the theft of trade secrets. Prosecutors have charged Chinese workers in the United States with stealing Boeing aircraft information, specialty seeds and even the pigment used to whiten Oreo cookie cream.

Other researchers and academics are being closely watched. The FBI is investigating a Chinese-American mapping expert who abruptly resigned from Ohio State University last year and disappeared while working with NASA, The Columbus Dispatch reported this week. In May, the Justice Department charged a Chinese professor and others with stealing acoustics equipment from U.S. companies.

About a dozen FBI agents, some with guns drawn, stormed Xi’s home in the Philadelphia suburbs in May, searching his house just after dawn, he said. His two daughters and his wife watched the agents take him away in handcuffs on fraud charges.

"Unfortunately I think this is influenced by the politics of the time," Xi said. "But I think it’s wrong. We Chinese-Americans, we contribute to the country, to the national security, to everything."

Temple University put him on administrative leave and took away his title as chairman of the physics department. He was given strict rules about who at the school he could talk to. He said that made it impossible for him to continue working on a long-running research project that was nearing completion.

Xi, who came to the United States in 1989 and is a naturalized citizen, was adamant that he was innocent. But it was only when he and his lawyers reviewed the government’s evidence that they understood what had happened.

"When I read it, I knew that they were mixing things up," Xi said.

His lawyers contacted independent scientists and showed them the diagram the Justice Department said was the pocket heater. The scientists agreed it was not.

In a sworn affidavit, one engineer, Ward S. Ruby, said he was uniquely qualified to identify a pocket heater.

"I am very familiar with this device, as I was one of the co-inventors," he said.

Last month, Zeidenberg delivered a presentation for prosecutors and explained the science. He gave them sworn statements from the experts and implored the Justice Department to consult with a physicist before taking the case any further. Late Friday afternoon, the Justice Department dropped the case "in the interests of justice."

"We wish they had come to us with any concerns they had about Professor Xi prior to indicting him, but at least they did listen," Zeidenberg said.

Xi choked back tears as he described an ordeal that was agonizing for his family.

"I barely came out of this nightmare," he said.

© 2015 The New York Times Company