UNITED NATIONS —Prime Minister Siniora of Lebanon has asked Secretary-General Ban for assistance in investigating yesterday’s assassination of yet another lawmaker in his quickly thinning parliamentarian majority coalition. However, the Assad regime is adept at hiding its violent involvement in Lebanese politics, a source told The New York Sun.

The source, who says he is a former Syrian military officer and that his position placed him at “the heart of secret Syria,” said the father of the country’s current president, the late Hafez al-Assad, devised a system in which front men are presented as responsible for running Syria’s operations in Lebanon. In reality, only a small number of people with “unknown names” are “operating secretly,” the source said, creating “all the destruction” in Lebanon. He declined to identify the men.

The legislator killed yesterday, Antoine Ghanem, a member of the Christian Phalange party, was the fourth victim of a mysterious assassination campaign since the February 2005 killing of a former Lebanese prime minister, Rafik Hariri. The killers are targeting members of an anti-Syrian coalition that commands a slim majority in Parliament.

Yesterday’s powerful explosion in a Beirut Christian neighborhood killed at least six people in addition to Ghanem, and wounded 67. Condemnations were issued in Paris, Washington, and by Mr. Ban, who called the “brutal assassination” in Beirut a “terrorist attack.” In a letter, Mr. Siniora asked Mr. Ban for “technical assistance” from a commission that the U.N. Security Council set up after the assassination of Hariri, to investigate yesterday’s event. The former Syrian military officer spoke with the Sun over the last two days from an unidentified country and asked that details about his identity be concealed. He said the ruling elite in Damascus fears the U.N. investigators, who are expected to present their findings about the Hariri killing and related assassinations to an international tribunal soon to be set up in the Netherlands. But in his assessment, the source added, it will be hard for the investigators to get to the real perpetrators.

The U.S. Treasury Department recently froze the assets of the former Syrian security chief in Lebanon, Ghazi Kanaan, and his successor, Rustum Ghazali, who were also implicated by the U.N.commission’s investigators. But according to the source, these two men may know little of the real goings-on.

In the secret operations system that was devised by the late Assad, according to the source, men like Messrs. Kanaan and Ghazali are in the dark about the true operations Damascus runs in Lebanon. They are “the last persons [Damascus] would inform before an operation” like the Hariri assassination, he said.

Therefore, if the current U.N. investigator of the assassination, Serge Brammertz, tries to prove their culpability, “he will not find anything,” the source said. Nevertheless, he added that he is sure Syria is behind the killings, operating through a secret structure answerable directly to President al-Assad and his sister, Bashra Shawkat, whose husband, Assef, was also implicated by the U.N.commission.

After yesterday’s explosion, the coalition representing Lebanon’s Maronite Christian, Druze, and Sunni communities, controls 68 seats in Parliament, while 59 seats are controlled by pro-Syrian opposition parties of Shiite and Christian legislators.

At stake is Lebanon’s presidential election, which begins next week. The presidency is reserved in the constitution for a Maronite Christian. The staunchly pro-Syrian President Lahoud is expected to step down in November.

Syria’s allies in Parliament, which include Hezbollah-affiliated legislators, are hoping that a former general, Michel Aoun, who is currently allied with Damascus, will win the presidency. They are vowing to block anti-Syrian candidates in Parliament by withdrawing their votes. At least 85 members of Parliament need to be present to elect a new president.