BEIRUT, Lebanon, AP – Police arrested three former Lebanese security chiefs Tuesday at the request of the U.N. investigation into the assassination of former Prime Minister Rafik Hariri, Lebanese security officials said.

Chief U.N. investigator Detlev Mehlis wants to question the former security chiefs, who are pro-Syria, the officials said, speaking on condition of anonymity because of the investigation”s sensitivity.

The arrests were the first since Hariri and 20 others were killed in a massive bomb in Beirut on Feb. 14. The detainees have previously been questioned by Mehlis, the German prosecutor who is leading the U.N. Security Council-mandated investigation into the assassination.

The killing of Hariri, who was seen as being quietly opposed to Syria”s role in Lebanese affairs, provoked massive demonstrations against Syria and the pro-Syrian government in Lebanon. International pressure led to Syria”s withdrawing its troops from Lebanon in late April, ending a 29-year military presence in the country.

All three former security chiefs detained had stepped down in April as calls mounted for their dismissal from Lebanese politicians opposed to Syria. Many Lebanese blamed Hariri”s assassination on Syria and pro-Syrian elements of their government. Syria and its Lebanese allies have denied any involvement.

Those arrested were: Maj. Gen. Jamil Sayyed, the former chief of General Security; Maj. Gen. Ali Hajj, the former director general of the Internal Security Forces; and Brig. Gen. Raymond Azar, the former director general of military intelligence.

Mehlis also asked for former legislator Nasser Qandil, a staunch supporter of Syria, to be detained for questioning, the officials said. But when police went to Qandil”s house in Beirut, he was not there and his wife said he was in Syria, the officials said.

A fifth person, Brig. Gen, Mustafa Hamdan, the pro-Syrian head of Lebanon”s Presidential Guards, was told he must come to Mehlis” office for questioning, the officials said. Hamdan has previously been questioned by Mehlis.