A powerful car bomb flung glass and heavy strips of metal across a wide intersection in downtown Beirut on Friday, killing a former cabinet minister and underscoring Lebanon’s growing instability as it absorbs the impact of neighboring Syria’s increasingly bloody civil war.

Mohamad Chatah, a senior aide to former prime minister Saad Hariri and a member of Lebanon’s Future Movement party, was killed in the blast. Chatah, a Sunni Muslim, served as Hariri’s finance minister and was Lebanon’s ambassador to the United States from 1997 to 2000.

Chatah’s assassination fueled fears that Lebanon may soon be forced to revisit its own civil war. The country has been unable to free itself from the sway of the Syrian conflict that is fast engulfing this turbulent region.

“You now have a Lebanon that will be engaged in a tit-for-tat to the extreme,” said Salman Shaikh, director of the Brookings Doha Center. “Whether that takes it into civil war, God only knows.” But Chatah’s killing marks “a dangerous forebear for how 2014 can unfold,” he said.

At least four other people were killed in the explosion and 75 were injured, Lebanon’s Health Ministry said. The blast, which shook the center of the capital around 9:40 a.m., ignited nearby cars and shattered the facades of high-end apartment and office buildings.

Minutes after the blast, the twisted wreckage of two cars — one of which soldiers said carried the bomb — sat smoking in pools of black water littered with pieces of human flesh. Glass and debris lay scattered across a nearly quarter-mile radius just blocks from Lebanon’s parliament and government buildings.

An employee in an adjacent office building, who gave his name only as Nader, said he rushed downstairs in panic after the blast. “The first thing I saw was half of a woman in the garden next to the trees, and then a man who had a piece of metal in his head, dead on the ground,” he said.

Paramedics and forensic investigators gathered body parts into bags and photographed a yard-wide crater in the street that soldiers said appeared to be the blast site. One soldier stood among a cluster of other officers, holding what appeared to be a portion of a human skull.

Chatah and the political party to which he belonged — Hariri’s Future Movement — were vocal opponents of the powerful Lebanese Shiite militant group and political party Hezbollah, which backs the regime of President Bashar al-Assad in Syria.

Chatah regularly criticized Assad and his backers, Hezbollah and Iran, and he tweeted cynical remarks about the diminishing potential for stability in Lebanon as he watched it become increasingly destabilized by the 21 / 2-year-old civil war in Syria.

The war has swept up regional powers, including Hezbollah, which has sent fighters to Syria to support Assad. Meanwhile, hundreds of thousands of refugees have spilled into neighboring countries.

Bombings and assassinations in recent months have targeted Assad’s allies and foes in Lebanon. A Hezbollah commander was killed this month, and a blast targeted Iranian officials outside Iran’s Beirut embassy last month.

Chatah was the latest of more than a dozen Hariri allies to be assassinated since 2005. But given that he was a moderate politician rather than a security official, his killing sends a far more potent message to the region at large, Shaikh said.

“It is clearly linked to this broader struggle that’s taking place in the region,” he said, adding that Syria’s regional spillover is well underway and that Washington should take note.

“Many of us have been saying this until we’re blue in the face — including to Obama administration officials: If you think you can contain the effects of Syria without really resolving [the crisis there], you’re going to be mistaken,” Shaikh said.

Secretary of State John F. Kerry issued a statement condemning what he called an “abhorrent terrorist attack and assassination.”

“I had the privilege of spending many hours with Chatah during my visits to Beirut as a United States Senator, and I know he was a voice of reason, responsibility and moderation,” Kerry said.

There were no assertions of responsibility for Friday’s explosion, but Chatah’s allies in the Future Movement swiftly blamed Hezbollah.

“Those who assassinated Mohamad Chatah are the ones who assassinated Rafiq al-Hariri, and who want to assassinate Lebanon and humiliate and weaken the State,” Saad Hariri said in a statement issued by his office.

Hariri’s father, former Lebanese prime minister Rafiq al-Hariri, was assassinated in a 2005 bombing that ignited popular opposition to Hezbollah and led to the expulsion of Syrian troops from Lebanon.

Five members of Hezbollah are set to go on trial at The Hague next month as the result of a four-year investigation by a U.N.-backed Special Tribunal for Lebanon. But none of the suspects are in custody.

Hezbollah denied any link to Friday’s explosion, calling the bombing “heinous,” according to the organization’s al-Manar television channel.

Minutes before the blast that killed him Friday, Chatah had tweeted in frustration about what he saw as Hezbollah’s continued abuse of power: “#Hezbollah is pressing hard to be granted similar powers in security & foreign policy matters that Syria exercised in Lebanon for 15 yrs.”

Speaking to a private Lebanese television channel hours after the blast, Defense Minister Fayez Ghosn described Chatah as “a friend of mine, who was known for being moderate and for his love of this country.”

Ghosn told Al Jadeed TV that he hoped Chatah’s death would motivate the citizens of this deeply divided nation to join together with “one policy: to talk to each other and to build a new Lebanon — for Lebanon cannot continue the way it is today.”

Others, including Chatah, have been less optimistic.

Sunni, Shiite and Christian groups backed by regional and international powers fought a devastating 15-year civil war that ended in 1990. Lebanon’s politics remain fraught with sectarian and regional tensions, its government staffed by caretaker ministers amid a political deadlock.

Nearly a million Syrian war refugees, most of them Sunni, have flooded into Lebanon in the past 21 / 2 years, drastically altering the demographics of an otherwise tiny country that has pinned its stability on its capacity to maintain a delicate sectarian balance.

On Christmas Eve, Chatah had lamented that a local Christian official’s calls for Lebanon to maintain its neutrality, the control of unlicensed weapons and loyalty to the state were unlikely to gain any traction in the country’s current political environment. The official’s pleas were “gifts Santa is unlikely to bring us, yet,” he wrote in a tweet.

The absence of justice years after Rafiq al-Hariri’s death has also underscored the weakness of authorities who Lebanese complain are consistently undermined by corruption, violence and intimidation.

Even as investigators in white jumpsuits and gloves photographed, bagged and tagged evidence at Friday’s crime scene, Future Movement officials and their supporters were casting doubt on the potential for transparent justice.

“The signatories of the message do not hide their fingerprints,” Hariri said in his statement, without naming Hezbollah. “They will continue on the criminal path, and will insist on dragging Lebanon into the abyss of discord, as long as there is, in Lebanon, some who provide cover for those crimes.”

Liz Sly in London, Suzan Haidamous and Ahmed Ramadan in Beirut and Anne Gearan in Washington contributed to this report.