WASHINGTON — The Justice Department this week dropped prosecutions of five foreign researchers accused of concealing ties to China’s military and who were arrested last year in a highly publicized sweep against alleged Chinese spying in the United States.

The moves came as one defendant, cancer researcher Tang Juan, was scheduled to stand trial on Monday in the Eastern District of California on charges of visa fraud. A court on Friday granted the department’s motion to dismiss her case. Motions to dismiss are pending in the other four.

Documents filed in Tang’s case this week, including an FBI analysis, raised questions that could not be resolved before the trial, officials said. And several weeks ago, a judge in Sacramento dismissed a portion of her case — a false statements charge — after finding that FBI agents had not properly informed Tang of her right against self-incrimination.

All five defendants were charged with visa fraud.

Senior department officials insist that dropping the cases is not a reflection of a lack of evidence.

“This is not the same as saying that we were wrong to charge the case a year ago or that the proof was insufficient,” said one senior official, speaking on the condition of anonymity under ground rules set by the department. “In fact, I think the cases have done a lot to advance our deterrent objectives.”

The official pointed to an exodus of more than 1,000 researchers, who department officials said had apparently hidden their ties to the Chinese People’s Liberation Army and fled the United States after the arrests last summer.

The cases were part of a high-profile initiative that the Justice Department says reflects a strategic priority of countering Chinese national security threats, particularly in the realm of economic espionage.

The arrests last year were part of an enforcement strategy that extends to what the department calls “non-traditional collectors’‘ — or university and lab researchers who officials say are “coopted” into stealing American technology for China.

They came shortly after the State Department ordered the Chinese Consulate in Houston to be shut down, accusing it of being a hub for illegal spying and influence operations in the United States. The closure escalated tensions already heightened by U.S. displeasure with a strict new Chinese security law for Hong Kong and U.S. criticism of Beijing’s handling of the coronavirus pandemic.

Malcolm Segal, a lawyer who represented Tang, said he was pleased with the dismissal. He said Tang is returning to China to be reunited with her husband, mother and child.

“In all of our prosecutions, the Department of Justice evaluates the merits of a case as it prepares for trial,” spokesman Wyn Hornbuckle said in a statement. “Recent developments in a handful of cases involving defendants with alleged, undisclosed ties [to the PLA] have prompted the department to re-evaluate these prosecutions, and we have determined that it is now in the interest of justice to dismiss them.”

Prosecutors alleged that Tang, a researcher at the University of California at Davis, lied on her visa application, saying she had never served in the Chinese military, when in fact she was a uniformed PLA officer.

Tang, who initially sought refuge at the Chinese Consulate in San Francisco, was detained for about 50 days before being released on bail. Two other defendants were detained for about a year, and a fourth for about nine months, officials said.

The senior official said that the punishment for visa fraud typically does not exceed a year. That fact, combined with the prospect of prolonged litigation in several instances, led officials to assess that the interests of justice were best served by dropping the cases, the official said.

“At some point you ask what’s the right thing to do?” the official said. “And after a year given that we’ve achieved our objectives, rather than seek an adjournment of the trial, this was the right thing to do. And once you reach that judgment, in one case, it was logical to extend it to the other cases.”

The other cases involve defendants Guan Lei in the Central District of California, Xin Wang and Chen Song in the Northern District of California, and Kaikai Zhao in the Southern District of Indiana.