LISBON, Portugal (AP) — Police in Portugal searched premises across the country Monday, acting on a tip-off that Portuguese troops stationed with a United Nations force in Africa have smuggled diamonds, drugs and gold back into Europe.

Soldiers are suspected of smuggling the illicit material on military cargo planes traveling between Central African Republic and Portugal, the office of the Portuguese Armed Forces Chief of Staff said in a statement.

Hundreds of elite Portuguese troops, including paratroopers and commandos, have been stationed with the U.N. force in Central African Republic in recent years.

The Portuguese force commander on that mission was told in December 2019 about the possible involvement of Portuguese soldiers in diamond trafficking, the statement said, without providing further details of the tip-off.

After Portuguese judicial authorities were informed, an investigation began into whether troops were working as mules to smuggle diamonds, drugs and gold back home according to the statement.

Police said in a statement that more than 300 officers took part in the operation targeting “a criminal network, with international links.”

Police conducted searches of 100 sites, mostly homes, and served 10 arrest warrants, the statement said without elaborating.

The military has stepped up checks and inspections of military flights from Central African Republic.