"The team has returned from Damascus. They are set to meet tomorrow to assess their work in Syria and consider the next step," Nejib Friji, UN spokesman in Beirut, said.
In Damascus, a Syrian foreign ministry official said the UN team had ended its visit and called Syria's cooperation with the investigators a sign of Syria's willingness to help them "to unveil the perpetrators of this horrendous crime".
Neither Friji nor the Syrian official commented on the questions raised during the meetings.
Head of the UN team Detlev Mehlis also handed control of the crime scene back to Lebanese officials, Friji said, after foreign experts concluded their investigations.
The spokesman said it was now up to the Lebanese authorities when to reopen the road where a large truck bomb killed al-Hariri and 20 other people on 14 February.
Neither side has named the officials interviewed but Lebanese political sources have said the at least eight officials questioned include senior intelligence officers working in Lebanon at the time and their commanders in Syria.
Many people in Lebanon have blamed Syria, the dominant power in the country for nearly three decades, for the bombing.
Rafiq al-Hariri was killed in a car
bomb blast in February
Syria, which denied any role in the murder of its former ally al-Hariri, agreed last week to allow Mehlis' visit after being accused of not cooperating with the investigation, which began in June. Mehlis came to Syria on Tuesday and left for Lebanon later in the day leaving his team to conduct the investigation.
According to the sources in Beirut, those interviewed included Rustom Ghazali, who was chief of intelligence in Lebanon, his predecessor, current Syrian Interior Minister Ghazi Kanaan and their top aides Mohammad Khallouf and Jamae Jamae as well as more senior officers.
Al-Hariri's killing sparked mass anti-Syrian protests in Beirut that forced Damascus to bow to world pressure and end its military presence in Lebanon in April.
Lebanon has charged three former security chiefs and the current head of its Republican Guard with murder in connection with the assassination. Their lawyers say they are innocent.
Four more people have been also arrested over the case. They are accused of fraud and withholding information after selling phone cards believed used in communications on the day of the bombing.