Asked by the Arab daily Al-Sharq al-Awsat if he was "perfectly convinced of Syria's responsibility in the murder of al-Hariri," Detlev Mehlis, a German magistrate, said "yes".
"The Syrian authorities are responsible," said Mehlis in the interview published on Saturday. He refused to go into detail.
On Thursday, Mehlis told CNN that there was an obvious link between the spate of assassinations to have hit critics of Damascus since al-Hariri's killing in February.
Damascus has consistently denied responsibility for any of the killings.
"We will have to look and we are looking for links between the assassinations as, pretty obviously, there are links between all these assassinations that happened after the death of al-Hariri," Mehlis said.
The UN investigator, whose mandate for al-Hariri's inquiry ended on Thursday, will stay on until a replacement is found.
Fayssal Mekdad is accused of
calling Gebran Tueni a dog
He released two reports into the assassination, in October and this week, which both cited evidence suggesting that Syrian and Lebanese intelligence officers were involved in al-Hariri's murder.
The second report coincided with the murder in a devastating car bomb attack of anti-Syrian MP and press magnate Gebran Tueni.
Tueni to sue
Meanwhile, Ghassan Tueni, the father of the slain MP, on Friday said he would sue Syria's ambassador to the United Nations for derogatory comments about his son.
The elder Tueni, a veteran Lebanese diplomat, accused Fayssal Mekdad, Syria's ambassador to the UN, of comparing his son to a "dog" in comments reported on Wednesday by the US daily, The New York Sun.
"I will sue him (Mekdad) before the American courts," Tueni, 79, said on Friday.
"So now every time that a dog dies in Beirut there will be an international investigation?"
The New York Sun, quoting Fayssal Mekdad, Syria's ambassador to the UN
In its report from UN headquarters in New York, the Sun quoted a diplomat who overheard a conversation between Mekdad and an Arab diplomat in which the derogatory comments were allegedly made.
"So now every time that a dog dies in Beirut there will be an international investigation?" the paper reported Mekdad as saying to a colleague during a closed-door session.
The diplomat who overheard and reported the conversation
declined to be named, the US newspaper said.
The Tueni-owned Al-Nahar newspaper said Mekdad had sent a letter to Tueni in which he "categorically denied" the comments attributed to him by the US daily.