HARARE, Zimbabwe (AP) _ Authorities in Zimbabwe turned down a U.S. offer to help investigate three Americans arrested on spying and weapons charges, the U.S. Embassy said today.

An FBI and a customs agent returned to their station in neighboring South Africa after spending a day in Harare, embassy spokeswoman Mary-Jo Furgal said.

``The Zimbabwe government turned down their offer of assistance,'' she said.

Three Americans purporting to be missionaries were arrested at the Harare airport March 7 after a gun one of them was carrying activated a metal detector.

That led to the discovery of a truck loaded with automatic weapons, rifles, telescopic sights, shotguns and handguns in the airport parking lot.

John Lamonte Dixon, 39, Gary George Blanchard, 34, and Joseph Wendell Pettijohn, 35, of the Indianapolis-based Harvestfield Ministries group are accused of terrorism, espionage and sabotage. No hometowns were given.

They are being held at Chikurubi maximum security prison outside Harare and are scheduled to appear in court March 26.

Furgal said U.S. consular officials visited the prison Wednesday after being denied access on Tuesday.

The embassy has denied any U.S. government links with the three Americans, which claims to have carried out missionary work in southeastern Congo.

Zimbabwe alleges the group were spying on Congolese and allied forces, including Zimbabwe troops fighting rebels in the eight-month Congolese civil war. They also allegedly were planning assassinations and sabotage in Congo and Zimbabwe.

The Americans say they were tortured by Zimbabwe police after their arrest.

Jeremy Callow, a lawyer in Harare acting for Harvestfield Ministries, said private doctors had still not been allowed to see the suspects by late Wednesday.