Jamil Sayyid, head of the powerful General Security Department, lashed out on Thursday at opposition demands for the resignation of security chiefs, indicating they will not step down.
He said was starting legal proceedings to clear his name and those of other security agency heads.
The government and its Syrian backers have been accused by the opposition of being behind the 14 February assassination of Rafiq al-Hariri.
The opposition and the al-Hariri family have called for an international investigation and the resignation of five security agency chiefs and the prosecutor general, alleging negligence.
The opposition is calling for resignations for political reasons, Sayyid told a news conference. "Do not mix politics with crime. ... Let justice decide."
"All the heads of security institutions are ready for trial and accountability," Sayyid said. "We have no secrets to be embarrassed of."
"I have decided on behalf of the commanders without consulting them ... to sue ourselves through gathering all the information touching on the institutions ... so that responsibilities are determined," he added.
"All the heads of security institutions are ready for trial and accountability"
Head of the General Security Department
Sayyid, a retired army major-general who has been in the post since 1998, is considered one of the most influential security chiefs in the country. His General Security Department, which is part of the Interior Ministry, is in charge of border points, issuing passports and censorship of publications.
But since his appointment by Lahud, the department has taken on additional significance, becoming the nerve centre of the Lebanese security and intelligence network.
Before moving to the General Security Department, Sayyid was deputy head of Lebanese military intelligence.
For their part, Lebanese opposition members scoffed at Sayyid's statement. "They should stop underestimating people's intelligence," Walid Aido, a member of al-Hariri's parliamentary bloc, who is a former judge, said.
Farid Khazin, an opposition lawmaker, asked: "How are you going to interrogate him (Sayyid) when the evidence is with him?"
Last month, Sayyid sued the editor-in-chief of the Kuwaiti al-Siyasa newspaper, Ahmad al-Jar Allah, for defamation. In a report on al-Hariri's death, the newspaper had named Sayyid, Syria's chief of military intelligence Gen Asif Shawkat and another Syrian intelligence official, Maj-Gen Bahjat Sulaiman.
Sayyid said the report, which has circulated on the internet, had harmed his reputation.