A British businessman facing extradition to the United States on allegations of trying to procure military equipment for China is feared to have fled Serbia for China after escaping house arrest alongside a suspected Chinese handler, The Mail on Sunday reported.

John Miller, 63, and Cui Guanghai, 43, were arrested in Belgrade on April 24 following an FBI sting operation, according to court documents and the newspaper. They were taken into custody at the Hyatt Regency Hotel hours before they were due to fly to Beijing and were placed under house arrest in separate rented apartments in the Serbian capital while U.S. authorities sought their extradition.

A Belgrade court issued arrest warrants for the two men on Aug. 15 after, the newspaper reported, both removed their electronic surveillance tags and disappeared. It was not immediately clear when or how they left Serbia, or whether they had assistance, and there was no immediate confirmation from Serbian, British or U.S. authorities about their current whereabouts.

Documents filed in the U.S. District Court for the Eastern District of Wisconsin in Milwaukee allege Miller, a recruitment specialist from Tunbridge Wells, Kent, sought to obtain military hardware in the United States for the People’s Liberation Army. The filings, as described in reporting, say the efforts included attempted purchases of missile launchers, air-defence radars, so-called Black Hornet microdrones and an NSA-approved hand-held device for secure transmission of classified information. The FBI said intercepted communications indicated Miller was aware he was acting under the direction and control of the Chinese government, the newspaper reported.

Miller has been charged in the United States with smuggling, stalking and violations of the Arms Export Control Act. If convicted, the charges carry a potential sentence of up to 40 years in prison, according to the court filings cited by the Mail.

Cui, a U.S.-based Chinese national and an Edinburgh University graduate, is described in reporting as a suspected senior Chinese intelligence official. The newspaper published images it said showed the pair together in London’s Chinatown and photographs of Cui and his wife in Edinburgh, Dublin and London. The Mail quoted family members saying Miller’s relatives believed he may have returned to China, where his son reportedly lives.

After the April arrests, British security services were understood to be liaising with the FBI, the reporting said. U.S. authorities, according to the court filings, had waited for Cui to travel to Europe before moving to effect the arrests in Belgrade.

Family members quoted by the Mail expressed concern about Miller’s disappearance. Alan Miller, identified by the newspaper as the businessman’s brother, said he believed the two men must have had help and that his brother was likely in China. A family friend said Miller’s wife told them she did not expect to see him again.

Serbian courts issued the arrest warrants in mid-August, but public details from Serbian police, Britain’s Home Office or the U.S. Department of Justice about the men’s escapes and current status were not available in the reporting. The allegations that form the basis of the U.S. case remain subject to legal processes in multiple jurisdictions.

The developments follow months of heightened scrutiny in Western capitals over allegations of Chinese intelligence operations targeting military technology. Authorities have in recent years pursued cases involving attempted procurement of sensitive equipment, and governments have warned of the national security risks posed by covert acquisition efforts.

The Mail on Sunday report is the primary source of details on the escape and family statements. Journalists sought comment from Serbian, U.K. and U.S. authorities, and from representatives for Miller and Cui, but the report did not include official confirmations of their current locations or of any cross-border movements after the Aug. 15 arrest warrants were issued.