By Gérard Bon and Claude Canellas
PARIS/BORDEAUX (Reuters) - Paris prosecutors launched an anti-terrorism probe on Thursday into death threats against a judge who put ex-president Nicolas Sarkozy under formal investigation in a party funding case, and into similar threats against several journalists.
Investigating magistrate Jean-Michel Gentil's decision about Sarkozy last Thursday raised hackles on the political right.
Many conservatives question the impartiality of judges given wide powers to conduct secret proceedings to investigate important criminal cases.
Sarkozy has rejected as unfounded the basis of the probe - that he exploited the mental frailty of France's richest woman to raise election funds.
His former speechwriter Henri Guaino has said Gentil has "dishonored the justice system" and "besmirched France".
A letter addressed to Gentil containing a large-caliber blank cartridge and a threat to "get you personally" arrived at the prosecutor's office in Bordeaux, where he is based, on Wednesday.
On Thursday, that office said it had opened a probe into the unknown group that signed the letter, the "Interaction of the Forces of Order".
The Paris prosecutor's office then ordered judicial police who focus on anti-terrorism cases to investigate, prosecutors told Reuters.
Without naming Sarkozy nor the funding scandal, the letter said Gentil was driven by ideology that had caused "irreparable harm".
The author said the group would "get you personally or, failing that, a member of your entourage".
"The bullets are blanks, until we take our action up a notch."
Two political journalists and the president of France's broadcasting regulator (CSA) also received threatening letters which will be part of the anti-terrorism investigation.
The letters sent to private radio Europe 1's Jean-Pierre Elkabbach, Michael Darmon of private television station iTele and the CSA's Olivier Schrameck also contained empty cartridges or blanks, a legal source said.
Although Schrameck was cabinet secretary to former Socialist Prime Minister Lionel Jospin, the choice of the two other journalists appeared to muddy suspicion that the group behind the letters was targeting leftist sympathizers.
The scandal for which Sarkozy is being investigated revolves around alleged payments made to his 2007 election fund.
Sarkozy has denied taking campaign funds from Liliane Bettencourt, France's richest woman who was declared to have dementia in 2006, but members of her staff questioned by Gentil and other magistrates have offered contradictory testimony.
(Additional reporting by Chine Labbe; Writing by Alexandria Sage, edited by Richard Meares)