Hezbollah, the Shiite military group and political party, has stood by President Bashar al-Assad of Syria, sending fighters to support his army. And many of Lebanon’s Sunnis sympathize with the Sunni-led rebels in Syria. Some have sent weapons to the rebels or have crossed the border to join them. The bombings in Lebanon have raised the specter of attacks here by militants linked to Al Qaeda. Al Qaeda is a Sunni organization that denounces Shiite Iran and the Lebanese group Hezbollah as heretics.

Lebanon’s army said Saturday that the detained militant, Majid bin Muhammad al-Majid, a Saudi who led a Qaeda affiliate based in Lebanon, known as the Abdullah Azzam Brigades, died of kidney failure in a military hospital. His arrest was announced last week.

Mr. Majid’s group claimed responsibility for a double suicide bombing near the Iranian Embassy in Beirut in November that killed at least 23 people and wounded dozens.

A security official said that Mr. Majid, an international fugitive, had entered Lebanon two weeks before his arrest under a false identity to receive medical treatment for a kidney ailment. He was arrested outside Beirut and taken to a military hospital, where his condition made it hard for the authorities to interrogate him.

Also on Saturday, Lebanese officials said they believed that a suicide bomber who blew up his car last week in a neighborhood where Hezbollah holds sway was a 19-year-old Lebanese citizen from an area near the border with Syria.