A bomb ripped through a bus carrying civilians and members of the military during Wednesday morning rush hour in the northern Lebanese city of Tripoli, killing 18 people and wounding 40, security officials said.
The officials said the dead included 10 off-duty soldiers.
The bomb was planted on the side of a main street and went off as the bus passed by. The streets were filled with people heading to work, which contributed to the many casualties, the officials said.
The military had no immediate comment. The security officials spoke on condition of anonymity because they were not authorized to speak to the media.
The blast raised suspicions that al-Qaida-inspired Islamic militants may have sought revenge on the military for its assault last year on the nearby Palestinian refugee camp of Nahr el-Bared, a one-time bastion of the Fatah Islam group. The monthslong battle killed hundreds and eventually drove out Fatah Islam.
But some local media speculated Wednesday's blast may be aimed at undermining a visit later in the day by the Lebanese president to Syria to patch up stormy relations between the neighbors.
Shattered glass littered Banks Street in Tripoli's center, as soldiers and policemen cordoned off the area to keep onlookers away and to investigate.
The small public bus, which had been bringing passengers from the remote northernmost Akkar region, home to many military members, was riddled with shrapnel. Soldiers used sniffer dogs to search nearby parked car, as forensic experts in white uniforms, face masks and gloves sifted through the wreckage of the bus picking up evidence.
Site of sectarian clashes
Tripoli, 53 miles on the Mediterranean coast north of Beirut, is Lebanon's second-largest city with a mostly Sunni Muslim population, dominated by groups loyal to the Western-backed parliament majority.
It has witnessed sectarian clashes between Sunni fighters and followers of the Alawite sect, an offshoot Shiite sect, in the past weeks that killed and wounded dozens of people.
Former Prime Minister Omar Karami, a prominent Tripoli politician, said it is too early to know the motive, but said the attack could be linked to the 2007 Nahr el-Bared violence, given the high casualties among soldiers.
Fatah Islam has claimed responsibility for a bomb blast that killed a soldier in Abdeh, near Tripoli, on May 31.
Lebanon has seen a series of explosions in the last 3 1/2 years, including the 2005 truck bombing that killed former Lebanese Prime Minister Rafik Hariri in Beirut, an explosion that sparked the political and security upheaval in the country.
Anti-Syrian Lebanese have blamed most of the bombings on Damascus, since many of them targeted anti-Syrian figures. Damascus has denied the claims, and no one has ever been arrested in the bombings. There have been no serious attacks on politicians or public places since February.
The latest violence comes at an especially sensitive time for Lebanon.
On Tuesday, parliament approved a national unity government that gives the Iranian- and Syrian-backed Hezbollah opposition a more powerful say in the running of the country, including veto power over major decisions.
The explosion also comes as President Michel Suleiman holds a landmark visit to neighboring Syria Wednesday — the first visit by a Lebanese president in about three years.
Ties have deteriorated since Syrian troops withdrew from Lebanon under international pressure in the wake of the Hariri assassination. Hariri's supporters blame Syria for the killing, while Damascus denies involvement.