War crimes prosecutor Luis Moreno-Ocampo arrived in Tripoli on Wednesday as part of an investigation into charges against Muammar Gaddafi's detained son, Saif al-Islam, sought for trial by the International Criminal Court (ICC).
The Hague-based court issued an arrest warrant for Saif al-Islam last year, after prosecutors accused him and others of involvement in the killing of protesters during the revolt that eventually toppled his father.
Libya says he will be tried in his home country but it has been unable to prize him out of the hands of the militia fighters who caught him in the southern desert in November.
Saif al-Islam remains in a secret location in the western town of Zintan.
Upon arrival at Tripoli airport, Moreno-Ocampo told reporters: I'm here because I am still investigating crimes.
Asked whether a potential deal was being brokered with the Libyan government about trying Saif al-Islam in Libya under the supervision of the ICC, he said: I am a prosecutor at the ICC, I don't make deals. We apply the law.
The judges of ICC ordered (Libya) to surrender Saif. The Libyan government says they will challenge the admissibility of the case before the end of April and then the judges will decide.
The ICC this month rejected Libya's request to postpone handing over Saif al-Islam to face war crimes charges. It ordered Tripoli to comply with its obligations to enforce the warrant of arrest and surrender him without delay.
Libya has appealed the decision.
Ahmed al-Jehani, a Libyan lawyer in charge of the case and who liaises between the Libyan government and the ICC, said he was optimistic Moreno-Ocampo's visit would help convince judges to allow the North African country to try Saif al-Islam locally.
Ocampo's visit is part of a continued relationship between Libya and the International Criminal Court, he told Reuters. When the judges hear the appeal of the Libyans to have Saif tried locally, they will also hear Ocampo's point of view.
A U.N. Security Council Resolution obliges Libya to cooperate with the court, the ICC says, and Tripoli's failure to hand him over could result in it being reported to the Council.
Libya's government wants to transfer him to Tripoli and put him on trial there. He faces the death penalty if found guilty by a Libyan court but a prison term if convicted by the ICC.
But the government remains in deadlock with the Zintan fighters who refuse to hand him over.
A delegation of the ruling National Transitional Council (NTC) travelled on Monday to the western town, 160 kms (100 miles) west of Tripoli, to try to broker a deal but left empty-handed.
Local council spokesman Khaled Ahmed al-Zintani said that the delegation left Zintan without a solution because fighters in Zintan believe the government cannot secure Saif al-Islam and are worried he would escape if brought to the capital.
He said they called on tighter security in Tripoli before any eventual transfer as well as salaries for their efforts.
We want the government to provide financial support for those who have been protecting Saif. It's their right, he said.
Zintan officials have also called on Saif al-Islam being tried in their straggling mountain town.
We want to try Saif al-Islam in Libya and we will not surrender him to any foreign entity, Zintan's military council head Bubakir Mohammed, who met the NTC delegation, said.
Libyans should decide where to try him and Zintan is capable of doing so.
Moreno-Ocampo said he would also travel to the coastal city of Misrata during his trip to Libya. I am going to Misrata ... because we have a mandate to investigate all the crimes committed here so we have to see what Libya is investigating.
Former rebel fighters in Misrata are holding some Gaddafi aides in prisons but it was not immediately clear what Moreno-Ocampo planned to visit while in the city, east of the capital.
(Additional reporting by Ali Shuaib; Writing by Marie-Louise Gumuchian and Hadeel Al-Shalchi; Editing by Robin Pomeroy)