Ousted Lithuanian President Rolandas Paksas is being investigated for leaking state secrets, prosecutors say.
Paksas is fighting to return to the presidency
No details were given, but Mr Paksas said he was suspected of telling someone that his telephone had been tapped by the intelligence services.
Correspondents say the person in question is likely to be the Russian businessman, Yuriy Borisov.
Mr Borisov is accused of involvement in the illegal arms trade - an allegation which led to Mr Paksas' impeachment.
"I am suspected of letting it be known that one person's telephone conversations were under surveillance," the ousted leader told reporters after being informed of the continuing investigation against him.
The former president has registered to stand for re-election in June.
Mr Paksas has accused his political foes of waging a "vendetta".
On 6 April he became the first European president to be impeached when parliament ruled that he had abused his powers to aid a Russian political backer and influence the outcome of an unrelated privatisation.
Denying the charges against him, Mr Paksas, a flamboyant 47-year-old former sportsman, has vowed to stand again for election in accordance with the Lithuanian constitution.
At the 2003 election, he won a shock victory over the incumbent, Valdas Adamkus.