Lebanese police on Tuesday detained nine men, including current and former high-ranking security officials closely linked to Syria, for suspected involvement in the assassination in February of former prime minister Rafiq Hariri.

At dawn, police and U.N. investigators raided the houses of three former security chiefs and a former member of parliament, and later detained the commander of the presidential guards, his brother and three other senior security officials, authorities said.

The police action marked a decisive moment in the U.N.-mandated investigation of Hariri's death, led by Detlev Mehlis, a German prosecutor who indicated last week that Syria had not cooperated with his work. U.N. investigators were expected to release their findings to the Security Council before a Sept. 15 deadline.

Given the men's long-standing ties to Syria, the detentions suggested the investigators were examining at least an indirect Syrian role in either planning or carrying out Hariri's assassination.

At the United Nations, a U.S. diplomat noted the potential Syrian connection in the detentions.

"This is a very dramatic development," said Anne W. Patterson, the deputy ambassador. "These gentlemen who have been arrested do have ties reportedly, have long-standing ties with Syria."

In Washington, the State Department welcomed the arrests as part of what it called progress in naming Hariri's assassins. "These arrests, as well as these further questionings, are part of that effort," said Sean McCormack, a State Department spokesman.

Across the Lebanese political spectrum, officials predicted that the number of suspects would soon increase.

"Undoubtedly there will be more to come," Hariri's son, Saad, a member of parliament, told al-Arabiya television from Paris, where he said he had fled after receiving threats. "What's important is that we have to know who killed Rafiq Hariri."

Hariri was killed Feb. 14 in a massive bombing as he drove along a Beirut street that overlooks the Mediterranean Sea. Many in Lebanon blamed the assassination on neighboring Syria, citing Hariri's mounting criticism of the Syrian presence in Lebanon as a potential motive. The Syrian government has denied any role, but within days of the killing, it faced widespread criticism in Lebanon, which it had dominated since 1976 through the presence of thousands of troops.

Syria withdrew its troops in April after international pressure and protests that, at one point, involved hundreds of thousands of people. The resulting popular euphoria has since faded, following a series of bombings in Beirut and the assassination of two other anti-Syrian figures. In past weeks, Saad Hariri and other politicians have fled the country, fearing they were named on a widely rumored hit list that includes some of the country's most senior leaders.

Prime Minister Fuad Siniora said U.N. investigators had requested the detention of the former security chiefs -- Brig. Gen. Jamil Sayyid, the former chief of General Security; Brig. Gen. Ali Hajj, the former director general of the Internal Security Forces; and Brig. Gen. Raymond Azar, the former director general of military intelligence. Before the Syrian military withdrawal, the men worked closely with Syrian security officials, and their resignations were among the key demands of protesters after Hariri's killing. The security officials resigned in April and had been questioned earlier by Mehlis, the U.N. investigative chief.

Despite feuds with both Lebanon's prime minister and president, Sayyid was considered the most influential of the security chiefs. Many analysts described him as the country's most powerful figure, tracing his authority to Syrian President Bashar Assad and his father, Hafez Assad, the longtime president who died in 2000.

Another major figure among those detained was Brig. Gen. Mustafa Hamdan, the head of the presidential guards and the only pro-Syrian security official to keep a senior post after the assassination. Hamdan is considered possibly the closest aide to Lebanese President Emile Lahoud, the most powerful of Syria's remaining allies in the Lebanese government. Hamdan turned himself in Tuesday at the headquarters of the U.N. investigating team.

Another prominent suspect is Nasser Qandil, a former member of parliament, whose home was also raided. He was in Syria at the time but returned to Lebanon in the afternoon and, under a police escort from the border, surrendered after speaking to reporters. He said he was stunned that police and U.N. investigators wanted to detain him and blamed "foreign intelligence apparatuses for providing false reports that involve me."

Lahoud, a former general, spoke in defense of the men, and Hamdan in particular, noting that they were only being questioned. Under Lebanese law, the men can be held for four days without charges, although the detention order can be extended. The men were scheduled to appear before a Lebanese judge Wednesday, the Lebanese national news agency reported, possibly to face charges.

"Everybody is innocent before the law until proven guilty," Lahoud said. "If evidence showed up that they are guilty, then they might be put on trial."

The others detained were Hamdan's brother, Majed, and three current officials: Brig. Gen. Khodr Tawil and Brig. Gen. Faisal Rashid, senior state security authorities, and Gen. Jamal Mawas, a senior officer in the presidential guards, security officials said on condition of anonymity.

Staff writers Robin Wright in Washington and Colum Lynch in New York contributed to this report.

In Sidon, Lebanese carrying posters of former prime minister Rafiq Hariri celebrate the detentions in the investigation of his assassination. Brig. Gen. Raymond Azar was the director general of military intelligence. Nasser Qandil, a former Lebanese legislator, was escorted from Syrian border. Brig. Gen. Mustafa Hamdan is commander of the presidential guards. Brig. Gen. Ali Hajj was the director general of Internal Security Forces. Brig. Gen. Jamil Sayyid served as chief of General Security.