Prosecutors have cast him as the mastermind behind a plot to smuggle US-made parts through China and into Iran, where they were used to help that country's nuclear program.
But his defense lawyers say that Sihai Cheng was "low-hanging fruit" in a scheme designed by an Iranian national who now will be free from any prosecution at all while Cheng remains jailed.
The scheduled sentencing of the Chinese national follows a prisoner-release deal between the United States and Iran that has reverberations in Boston, outraging Cheng's lawyer.
"Mr. Cheng continues to be caught in the middle between Iran and the United States and China," Cheng's lawyer, Stephen Weymouth, said in court records. "The United States government is and has been treating Sihai Cheng unfairly, especially when viewed in the context of how the United States government has been treating other individuals who were either involved in this activity or have been involved in similar conduct."
Weymouth has asked that Cheng be sentenced to time served, or 24 months in prison, arguing that Iranian national Seyed Abolfazl Shahab Jamili is the true culprit in the smuggling plot.
"There is no doubt that this defendant [Jamili] was the leader of this criminal activity," Weymouth said. "Jamili orchestrated the entire process."
Cheng, Jamili, and two Iran-based companies were indicted in 2013 on charges of illegal exports to Iran for shipping US-manufactured pressure transducers to two Iranian companies. Those companies supplied the products to Kalaye Electric Company, an arm of the Iranian government that helped the country advance its nuclear weapons program.
The transducers, or pressure monitoring sensors, are used in gas centrifuges to enrich uranium and produce weapons-grade uranium.
Prosecutors say that Cheng directed the scheme by smuggling the transducers, as well as other equipment — such as centrifuge cascading piping and components needed for Iran's heavy water production plant — through businesses in China and into Iran.
The transducers are heavily regulated: They cannot be brought into Iran, and a special license is needed to bring them into China.
The parts were manufactured by Andover-based MKS Instruments, though that company was not targeted in the investigation, and its officials cooperated with authorities.
Cheng was arrested in 2013 when he traveled to England and was extradited to the United States. Jamili remained a fugitive, and Weymouth said authorities never truly sought to locate him.
But on Jan. 16, US Attorney Carmen M. Ortiz notified the court that the government would dismiss the charges against Jamili "based upon issues regarding securing extradition of the defendant and significant foreign policy interests."
According to court records, Jamili is one of 14 Iranian nationals who the United States has agreed to lift arrest warrants for as part of a prisoner release agreement the country reached with Iran earlier this month. The agreement has been touted as an example of the new working relationship the United States has with Iran since the countries reached an agreement last year to rein in Iran's nuclear proliferation efforts.
The United States has said that those whose charges have been erased were accused of violating US sanctions in Iran, and were not accused of terrorism.
However, prosecutors have sought to classify Cheng's crimes as terrorism.
Weymouth argued in court records that Cheng should be treated no differently, and that it would be "not fair for the government to prosecute Sihai Cheng but not to prosecute Jamili.
"Jamili orchestrated the entire process," he said.
Prosecutors have asked that Cheng be sentenced to 15 years in prison, saying he sold the parts, and that Jamili was his customer.
"The prosecution of individuals who violate our export laws and regulations as well as US sanctions is just as important now, if not more, than it was before the agreement was reached with Iran," prosecutors said in court records.
Cheng, 36, is scheduled to appear for a sentencing hearing Wednesday morning before US District Chief Judge Patti B. Saris.