Detlev Mehlis, a U.N. investigator probing the murder of former Lebanese prime minister Rafiq Hariri, has done what no Lebanese crime fighter could do: He has penetrated Syria's intelligence service, ordered the arrest of pro-Syrian generals and pursued suspects high up in Syrian President Bashar Assad's inner circle.

The German prosecutor's inquiry has jolted Syrian political ranks and made Mehlis a hero among Syria's opponents in Lebanon, who are struggling to shake the last vestiges of Syrian domination over the country's political life. It has also made Mehlis, 55, the target of death threats.

For those who have followed Mehlis's career, his hard-charging investigation into the deaths of Hariri and 22 others in a Beirut car bombing Feb. 14 comes as no surprise.

Mehlis is considered one of Europe's toughest anti-terrorism prosecutors, investigating complex international crimes, including the 1986 bombing of the La Belle discotheque in Berlin, which killed two American soldiers and a Turkish woman. A German court ruled in 2001 that the attack was planned by Libya's secret service; four people were convicted, including a former Libyan diplomat.

U.N. Secretary General Kofi Annan's May appointment of Mehlis, whose hiring was strongly supported by the United States and France, Syria's toughest Security Council critics, signaled that the United Nations was prepared to go head to head with Syria's ruling elite. But it also fueled concerns that Mehlis's sleuthing could trigger a violent reaction from implicated Syrian officials, pitch Lebanon into a new era of civil war and derail its quest for independence.

"This guy has a reputation: If you did something wrong, this is not the prosecutor you want to draw," said Flynt Leverett, a specialist on Syria and the Middle East who served on the National Security Council from March 2002 to March 2003. "The fact he was selected I don't think is coincidental. I think the United States was really pushing to have an investigation that could potentially open up the Assad regime."

Mehlis's investigation has provided an unprecedented challenge to Syria's leaders, who have ruled neighboring Lebanon with impunity since the 1970s, and has raised the possibility of high-level indictments. His mandate has been extended until Dec. 15.

"We think he has been very professional," said John R. Bolton, the U.S. ambassador to the United Nations. "We think that he follows the evidence where it leads. He doesn't overstate."

With the support of Lebanese officials, Mehlis has gained extraordinary access to wiretaps, telephone records and secret documents that portray an alleged conspiracy by Syria's top officials. Operating from the Monte Verde Hotel outside Beirut, he leads a team of as many as 100 international investigators, forensic experts and translators.

Lebanese authorities, acting on Mehlis's request, last month detained four pro-Syrian Lebanese generals who were suspected of participating in Hariri's assassination. And in one of the strongest breaks, a former Syrian intelligence officer questioned by Mehlis fingered several top Syrian and Lebanese officials by name, including Assad's powerful brother-in-law, Asef Shawkat, and Assad's brother, Maher.

Syria's ambassador to the United Nations, Fayssal Mekdad, said the charge against Assad's relatives is "a big lie," adding that Mehlis's findings lack credibility. He said the report will be used by powerful countries, principally the United States, to bully Syria.

Mehlis's performance has not been without missteps.

He hastily arranged a news conference Friday to counter suggestions that he had bowed to pressure from Annan to delete the names of prominent Syrian and Lebanese officials, including Assad's relatives, from his report.

The issue arose after U.N. officials inadvertently released an early electronic draft of the report, containing the officials' names, to select members of the Security Council. They were removed from the draft after Mehlis gave Annan a copy of the original report Friday morning. A time stamp on the file indicated the deletions had been made at about the same time as the meeting.

Mehlis insisted that Annan and other U.N. officials had not pressured him to sanitize the document. He said he removed the names after discovering the report was to be made public and because he did not have sufficient evidence to prove the individuals' guilt. U.N. officials in New York, however, had indicated weeks earlier that the report would be made public.

"No one influenced me, and I'm not one who would accept changes from outside," Mehlis said. "I can only tell you what I told you. If you believe it or not, it's up to you."

Lebanese observers say that Mehlis is nevertheless rewriting the history of the Middle East, by proving that even the region's most powerful rulers can be brought to account.

"Detlev Mehlis is a name that will be [written] in the history of the Middle East," wrote Raghida Dergham, a reporter and columnist for the London-based Arab language daily Al-Hayat. His investigation, she added, "will cause an earthquake in the whole Arab region."

Detlev Mehlis has been leading a U.N. inquiry that one observer said would cause "an earthquake in the whole Arab region."