Lebanese police are questioning several men after Wednesday's car bombing in Beirut which killed a general expected to become army chief, officials say.
Gen Hajj had been tipped to become head of the army
Gen Francois al-Hajj had been tipped for the post if MPs elected the current army head to the vacant presidency, in attempts to end a political crisis.
Pro-Western Prime Minister Fouad Siniora said the bombing was an attempt to derail the presidency vote.
Some of his allies blamed Syria and its Lebanese backers - a charge they deny.
At least three Lebanese nationals were arrested in the southern port of Sidon, officials said.
"They are being questioned in connection with the licence plate of a BMW car, found on the site of the blast," a police source was quoted as saying by the AFP news agency.
The army earlier said that a car packed with 35kg (77 lb) of explosives exploded as Gen Hajj drove past in Beirut's high-security suburb of Baabda. The blast killed Gen Hajj and his driver.
The vehicle's number plates were found to have been registered in the names of the detained men, officials said.
However, it remained unclear if the plates found by investigators at the scene of the attack belonged to the explosives-laden car.
Gen Hajj was chief of operations when Lebanon's army fought Islamic militants from Fatah al-Islam in Nahr al-Bared refugee camp earlier this year.
Feb 2005: Ex-PM Rafik Hariri
April 2005: MP Bassel Fleihan
June 2005: Anti-Syria journalist Samir Kassir
June 2005: Ex-Communist leader George Hawi
Dec 2005: Anti-Syria MP Gebran Tueni
Nov 2006: Industry Minister Pierre Gemayel
June 2007: Anti-Syria MP Walid Eido
Sep 2007: Anti-Syria MP Antoine Ghanim
Dec 2007: Army Gen Francois al-Hajj
But correspondents say it is questionable whether the militants would have the means to carry out such a precisely targeted attack.
They say the attack could further destabilise Lebanon, which is embroiled in its worst political crisis since its long civil war ended in 1990.
While the pro-West governing bloc and pro-Syrian opposition, led by Hezbollah, agree Gen Michel Suleiman should succeed former President Emile Lahoud to break a political impasse, the factions cannot agree on the formation of a new government.
Mr Lahoud left office on 24 November, and on Monday lawmakers postponed the presidential vote for an eighth time.
Critics have blamed Syria for a number of high-profile political assassinations in Lebanon in the past two years. Damascus has denied involvement.