Michael Campbell, of County Louth near Ireland's border with Northern Ireland, was convicted in Lithuania of attempting to buy and smuggle weapons for the Real IRA terrorist group, reports the Irish Times.
A Lithuanian judge sentenced him to 12 years in prison.
Campbell denies the charges. He claims he is a victim of entrapment after being caught as part of an elaborate sting operation involving British, Irish and Lithuanian intelligence agencies, the Irish Times says.
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Undercover officers posing as arms traders recorded Campbell negotiating the purchase of high-grade explosives, rocket-propelled grenades, AK-47s and a sniper rifle.
This video from the Guardian shows part of the footage played during the trial, in which Campbell discusses using explosives to boobytrap cars:
Other recordings revealed Campbell speculating about planting multiple timer-controlled bombs around London.
According to Sky News, Campbell was recorded saying:
"You imagine us getting over to England if you had 10 of them and 10 clocks in a holdhall. You imagine, with a six-hour timer we could be over to London and back. Just tick, tick, tick, tick... gone."
Chief prosecutor Irmantas Mikelionis said the evidence made it clear the arms were intended to be used for "terrorist attacks and [the] killing of innocent people in the UK".
Real IRA members were to be trained in Lithuania how to use the weapons, he said.
Prosecutors also want to try Campbell's brother for alleged involvement in the arms-smuggling plot, reports the BBC.
They are currently trying to extradite Liam Campbell, a suspected senior member of the Real IRA and one of four men found liable for the deadly 1998 Omagh bombing, from Belfast in Northern Ireland.
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