Serbian President Boris Tadic says the investigation into former Bosnian Serb army chief Ratko Mladic will be extended to anyone thought to have helped him avoid arrest for 16 years.
Mr Tadic says anyone who protected General Mladic will be prosecuted.
General Mladic would be transferred to The Hague to be tried for war crimes, despite an extradition appeal by his lawyers, the president insisted.
Arrested on Thursday he faces genocide charges over the 1992-95 Bosnian war.
However, General Mladic's legal team says he is in poor health and they will appeal on Monday.
They have requested that he be admitted to hospital over concerns about his health.
But Judge Maja Kovacevic has told reporters that General Mladic's health was good enough for him to be tried at the tribunal.
The man known as the butcher of Srebrenica was arrested in a village in northern Serbia on Thursday.
He faces genocide charges over atrocities in the 1992-95 Bosnian war, including the killings of 7500 Bosnian Muslim men and boys at Srebrenica in July 1995.
UN International Criminal Tribunal chief prosecutor Serge Brammertz said on Friday he was considering putting General Mladic on trial together with former Bosnian Serb political leader Radovan Karadzic.
Mr Karadzic was arrested three years ago and has been on trial since 2009.
The BBC reports any joint appearance would mean lengthy delays in the proceedings against him.
General Mladic is believed to have gone into hiding after the arrest of former Yugoslav President Slobodan Milosevic in 2001.
His wife Bosiljka and their son Darko visited him at the court on Friday.
Mr Saljic later said this was their first meeting with him in 10 years.
He told journalists his father was innocent and not in a fit state to be sent to The Hague.
Mrs Mladic recently said she thought her husband was dead.