The Argentine leader has given permission for sealed intelligence service files on the bombing of a Jewish centre in Buenos Aires in 1994 to be opened.
It is the latest surprise move by Nestor Kirchner, who despite being in the job for less than two weeks, has already announced a drastic shake-up of both the army and police.
The bomb completely destroyed the centre
Eighty-nine people died when a van stocked with explosives drove into the Israeli-Argentine Mutual Association (Amia) centre in July 1994.
"This is one of the most important pieces of news in nine years," said Abraham Kaul, head of the now rebuilt Amia centre, after a meeting with Mr Kirchner late on Thursday.
The latest decision in the case annuls a decree by his predecessor, Eduardo Duhalde, that sealed the files and prevented agents from the State Intelligence Secretariat (SIDE) from testifying.
Fourteen such agents are expected to do so now, including Hugo Anzorreguy, SIDE's head at the time of the bombing.
Correspondents say the documents might also provide clues to whether alleged blunders by the security forces before the attack were covered up by the authorities.
A presidential spokesman, Miguel Nunez, said Mr Kirchner had decided to "open the SIDE files on the Amia bombing and place them at the disposal of investigators".
He said the decision would "lift the veil of state secrecy from the functions of the SIDE".
The Lebanese militant group Hezbollah was first blamed for the attack.
Iran has also been accused of a role in the blast - which it vigorously denies.
But in the past nine years, the case has been plagued by disappearing witnesses and unexplained delays.
Argentina's 300,000-strong Jewish community is the largest in Latin America, and has been the target of other attacks.
A 1992 bomb attack on the Israeli embassy in Buenos Aires in which 29 people were killed also remains unsolved.
"For the first time in nine years, an Argentine Government has decided that the Amia cause is a cause of the Argentine state," said Mr Kaul.
"We have great expectations of this government."