German prosecutor Detlev Mehlis crossed the border from Lebanon to Syria in a heavily guarded convoy on Tuesday, an AFP photographer reported.
It is Mehlis's second visit to Syria this month and follows the arrest of four top Lebanese security men on murder charges about al-Hariri's assassination in a massive bomb blast on the Beirut seafront in February.
He is to quiz Syrian officials over al-Hariri's assassination that caused political upheaval in Lebanon and intense international pressure, which paved the way for Syria to end its 29-year troop presence there.
"Mr Mehli's visit to Damascus will last until the end of the week," the official Sana news agency reported on his arrival.
During a first trip to Damascus earlier this month, the Syrian Foreign Ministry agreed on the format for his questioning of Syrian officials.
The interviews will be a key test of the Syrian government's readiness to cooperate after the arrest of four top allies in Lebanon.
Interior Minister Ghazi Kanaan
is expected to be questioned
Government daily Tishrin said on Tuesday that Syria affirmed once again that it was committed to cooperating (in the inquiry) to arrive at the truth through legal means, tangible proof and in the context of international law.
"It is expected that the commission will interview Syrian witnesses so that they might assist in reaching the truth, as a result of their experience and the nature of their work," the paper said.
Mehlis is expected to question Interior Minister Ghazi Kanaan, a former military intelligence chief in Lebanon; his successor Rustom Ghazaleh, who left along with Syrian troops in April; and two key aides in Beirut, Mohammed Makhlouf and Jamaa Jamaa.
The four served as the main pillars of the security network Syria set up in Lebanon to consolidate its long domination.
Lebanon's central bank last week lifted the bank secrecy of the accounts of eight Lebanese and Syrian figures at the request of the UN investigation.
No Syrian suspects
Mehlis also plans to question Syrian President Bashar al-Assad's younger brother, Maher, who heads the Syrian presidential guard, the Kuwaiti daily As-Siyassa reported.
The German magistrate has insisted that no Syrian suspect has been identified, while also saying he believes there are "more people involved" than the four Lebanese security chiefs arrested so far.
Lebanese security chiefs are
being held for questioning
They are presidential guard chief Mustafa Hamdan, former general security boss Jamil al-Sayed, ex-internal security head Ali al-Hage and former army intelligence director Raymond Azar.
Syrian officials have pledged to answer all questions put to them about the bombing that killed al-Hariri and 20 others and was widely blamed on Syria and its allies in the then Lebanese government.
It was not known whether Damascus has already signed a judicial protocol with the commission, as was done by Beirut in a move that paved the way for the arrest of the security chiefs.