When I heard the news on Monday that a leading Beirut journalist had been killed, I knew who it was before they named him.

I had sat in Gibran Tueni's high-rise office last July, from which he ran Lebanon's foremost newspaper, An-Nahar. From his windows, we could look down at Beirut's port, rebuilt from the wreckage of civil war and a symbol of Lebanon's efforts to build a democracy free from violence. Tueni told me his life was at risk for opposing Syria's continuing efforts to manipulate Lebanon.

"Yes, there is a Syrian hit list," he said, meaning the Syrians had targeted their leading Lebanese critics. "We know this from foreign embassies and from inside," a reference to his own intelligence sources.

So I wasn't surprised that Tueni's armored SUV was blown up by a remote-controlled car bomb. But I was blindingly angry, because Lebanon is the one Arab country where democracy might be possible in the near term. The murder of Lebanon's best is meant to end this prospect.

The United States and the United Nations can't afford to allow these killings to continue unchallenged. Tueni was slain on the same day that a special U.N. investigator, Detlev Mehlis, was presenting a second report to the Security Council on the assassination of former Lebanese Prime Minister Rafik Hariri. This murder, in February, sent 1.5 million Lebanese into the streets; it aroused such international ire that Syria had to withdraw thousands of troops and agents from Lebanon.

But since the withdrawal, a string of prominent politicians and journalists, all critics of Syria, have been killed or injured by car bombs. "For the Syrians," Tueni told me, "anyone who can make his voice heard ... is a problem. The Syrians don't want to accept that things have changed in Lebanon, which was a gold mine of drug-dealing and money-laundering for them."

Mehlis' first report in October alleged that Hariri's killing was masterminded by the inner circle around Syrian President Bashar al-Assad. Monday's report said Syria had obstructed the U.N. probe and offered further "probable cause" that Syrian intelligence agencies directed Hariri's murder.

But the Syrians claim that Tueni's killing on the day of the new report shows someone is trying to frame them. Why would they want to provoke the United Nations, they ask, just as Mehlis is presenting charges that could lead to sanctions against them?

I can think of a couple of good reasons.

Despite masses of evidence, the Mehlis probe will have trouble making an airtight case against Syrian agents. Witnesses and their families have been threatened. In the murky Middle East, perpetrators of assassinations are almost never found.

One could easily imagine Syrian leaders thinking the U.N. case can never be proved. Even France, which strongly supported the probe, is backing off the threat of sanctions. Many critics of Syria (myself included) have worried that the collapse of the Assad regime might produce chaos and an Islamist resurgence.

So Syrian leaders may have concluded they can stick a finger in the United Nations' eye cost-free. Meantime, Tueni's death sends a strong warning to Lebanese that they cannot escape their Syrian big brothers. A car bomb — the modus operandi of Syrian intelligence — awaits those who challenge the Damascus line.

And there's another good reason one can imagine Syrians killing Tueni. The 47-year-old journalist stood for something unusual in the Middle East: the belief that pluralism can triumph in an area rife with sectarian violence.

Tueni had a Greek Orthodox father and a mother who was Druze (an offshoot of Islam). He considered himself Lebanese. He kept a cross on his desk and a Quran behind it. I once heard him declare that he was half-Christian, half-Muslim, causing a nearby group of Gulf Arabs to erupt in angry denunciation. They claimed one had to choose one's religious identity and couldn't pretend to bridge such divides.

Tueni was trying to change such thinking. He told me Lebanon had progressed beyond the civil war mind-set. I could imagine those who killed him wanting to warn that they will rekindle such strife if the Lebanese spurn the dons of Damascus.

Lebanon's prime minister asked the United Nations this week to broaden its investigation of Hariri's death to include the killing of Tueni and other anti-Syrian victims. He wants the case tried by some form of international tribunal to provide more protection to witnesses and judges.

Setting up that tribunal would be a fitting response to Tueni's killing. It would also help Lebanon fight off those who want to drag the country and the region down.