United Nations ? A U.N. investigation concluded that high-ranking Syrian and Lebanese security officials were involved in the assassination of former Lebanese Prime Minister Rafik Hariri, according to a report released Thursday.

The report by chief investigator Detlev Mehlis said Hariri’s Feb. 14 assassination was so complex that it would be difficult to imagine that the Syrian and Lebanese intelligence services didn’t know about it.

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

Mehlis wrote that the two nations’ intelligence services kept tabs on Hariri by wiretapping his phone constantly.

A Lebanese army soldier sits on top of an armored personnel carrier in front of a poster of slain former Prime Minister Rafik Hariri, left, and his son, legislator Saad Hariri, right, Thursday in Beirut, Lebanon. The Lebanese police and army took additional security measures as the country awaited the findings of a U.N. probe into the Feb. 14 assassination of Rafik Hariri.

The 53-page report said that the investigation is not complete and must be continued with Lebanese judicial and security authorities in the lead.

Several lines of investigation still need to be pursued, he 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 the Feb. 14 assassination of Hariri, a former prime minister whose motorcade was bombed on a Beirut street, killing him and 20 others. Syria has denied involvement.

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.

U.N. Secretary-General Kofi Annan gave the probe a three-month mandate when it began its work on June 16 but said it could be extended for three more months if necessary. In August, Mehlis received an extension beyond the original Sept. 15 deadline.