The exact role the FBI agents will be allowed to play in the investigation - as Miss Mack and her boyfriend remained behind bars as suspects - will be a secondary one to Indonesian police.
Under FBI 'rules' agents to not have the authority to make arrests in foreign countries, except in certain cases where they have the consent of that country’s government or police.
In the current case, police sources said, it is virtually certain that if Ms Mack and her boyfriend, who are facing murder charges, are found guilty they could face death by firing squad.
Senior police are understood to have told the FBI team that while neither Ms Mack nor Mr Schaefer have provided a motive for the killing they believe there are one of two reasons - or both.
As part of the FBI’s involvement, while giving way to the local police investigation, the US officers are expected to be taken to the St Regis Hotel and shown room 616 where Mrs Mack’s blood has been found.