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High-ranking Syrian and Lebanese security officials plotted the assassination of former Lebanese Prime Minister Rafik Hariri (search) in a complex operation that needs further investigation, a U.N. probe concluded Thursday.

The investigative report was the first official link between government officials in Damascus (search) and the car bomb that killed Hariri and 20 others on Feb. 14 and was almost certain to increase already heightened tensions in the region.

The decision to assassinate Hariri "could not have been taken without the approval of top-ranked Syrian security officials and could not have been further organized without the collusion of their counterparts in the Lebanese security services," the report said.

U.N. officials gave investigators two more months to pursue the probe and set Security Council (search) debate on the report for Tuesday.

Questions were also raised about Lebanese President Emile Lahoud (search), Syria's staunchest ally. He received a phone call minutes before the blast from the brother of a prominent member of a pro-Syrian group — a call that should be part of a further investigation, the report said.

The strongly worded report by chief investigator Detlev Mehlis didn't call for the arrest of any Syrians, but it was highly critical of the Syrian government. It accused Syrian authorities of trying to mislead the investigation, and directly accused Foreign Minister Farouk al-Sharaa of lying in a letter sent to Mehlis' commission.

Earlier this week, a U.S. official and two U.N. diplomats said the United States and France were preparing new Security Council resolutions critical of Syria over its alleged involvement in the assassination and alleged arms funneling to Lebanese militias.

U.S. Ambassador John Bolton (search) said shortly after the report's release that the United States has "considered various contingencies" but would decide what to do next only after it had read the report and consulted with "other interested governments."

Later, U.S. State Department spokesman Sean McCormack said: "An initial reading of the report indicates some deeply troubling findings and clearly the report requires further discussion by the international community."

The 53-page report painstakingly outlines Hariri's relationship with Lebanese and Syrian officials, and the events leading up to the assassination, which it said appeared to have been political. The report was based on the findings of an initial brief U.N. investigation, statements from 244 witnesses, crime scene exhibits, and the work of 30 investigators from 17 countries.

The report said the intelligence services of Syria and Lebanon kept tabs on Hariri before his assassination by wiretapping his phone, and there was evidence a telecommunications antenna was jammed near the scene of the car bomb.

The report quotes a Syrian witness living in Lebanon who claimed to have worked for Syrian intelligence in Lebanon as naming several officials who conspired to assassinate Hariri. They included Brig. Gen. Rustum Ghazale, the last Syrian intelligence chief in Lebanon who was in charge when Hariri was assassinated, and Brig. Gen. Mustafa Hamdan, who was Lebanese commander of the Presidential Guards Brigade at the time of the assassination.

Mehlis' team had already named Hamdan and three other Lebanese generals, all close to Syria, as suspects in the assassination, and Lebanon has arrested them.

Mehlis said Syria's cooperation in form — but not substance — "has impeded the investigation and made it difficult to follow leads established by the evidence collected from a variety of sources."

He called for the investigation to be extended with Lebanese judicial and security authorities in the lead.

"If the investigation is to be completed, it is essential that the government of Syria fully cooperate with the investigating authorities, including by allowing interviews to be held outside Syria and for interviewees not to be accompanied by Syrian officials," Mehlis said.

In a letter accompanying the report, U.N. Secretary-General Kofi Annan (search) said he would extend Mehlis' investigation until Dec. 15, which would allow the team to continue its work and help the Lebanese authorities.

Several lines of investigation still need to be pursued, Mehlis said. They include jamming devices in Hariri's convoy that were functioning at the time of the bombing. It appears there was interference with a telecommunication antenna at the crime scene at the time Hariri was killed in a massive car bomb, Mehlis wrote.

In Lebanon, authorities had increased security ahead of the report's findings. Many there blame Syria for Hariri's assassination. Syria has denied involvement and so have the four Lebanese generals.

Hariri's death led to demonstrations against Syria and magnified the international pressure on Damascus to withdraw its troops, which it eventually did. The Security Council approved a probe into Hariri's assassination on April 8.

The report said a Syrian witness living in Lebanon who claimed to have worked for Syrian intelligence in Lebanon told the commission that "senior Lebanese and Syrian officials decided to assassinate Rafik Hariri" about two weeks after the U.N. Security Council adopted a resolution in September 2004 demanding the withdrawal of Syrian forces from Lebanon.

The witness, who was not identified, claimed a senior Lebanese security official went to Syria several times to plan the crime. At the beginning of January 2005, a high-ranking Syrian officer posted in Lebanon told the witness that "Hariri was a big problem to Syria."

"Approximately a month later the officer told the witness that there soon would be an `earthquake' that would re-write the history of Lebanon," the report said.

Mehlis said the most likely scenario for the activation of the explosives was a suicide bomber. A slightly less likely possibility was a remote controlled device, he said.

Minutes before the bomb went off, Mahmoud Abdel-Al, the brother of Sheik Ahmed Abdel-Al, made a call to Lahoud's mobile phone and another to the mobile phone of Brig. Gen. Raymond Azar, then head of Lebanon's military intelligence. The brothers are both members of the pro-Syrian Al-Ahbash Sunni Muslim Orthodox group and the report called the sheik "a key figure in an ongoing investigation."