Ousman Sonko has been in custody since his arrest on January 26, 2017 in Switzerland, where he had applied for asylum after being sacked as The Gambia's interior minister.
He was arrested after a complaint by Geneva-based NGO Trial International, which believes he played a role in serious human rights violations, such as torture, particularly in places of detention.
Sonko had been one of Jammeh's top aides, serving in his presidential guard before heading the interior ministry.
He was fired in September 2016 after having held the post for 10 years.
Sonko has been repeatedly remanded in custody throughout the investigation by Attorney General of Switzerland (OAG) state prosecutors.
On November 1 last year, his detention was extended until January 25.
"We are about to close the preliminary procedure. We cannot provide any further information at this time," OAG spokeswoman Claudia Schon told AFP.
At the end of the investigation, prosecutors can issue an indictment or close the case. Several alleged victims have joined the case as civil parties.
"As always, the presumption of innocence applies," said Schon.
Jammeh held sway over the tiny West African state for 22 years until he was unexpectedly defeated in presidential elections in December 2016 by political newcomer Adama Barrow.
He then fled to Equatorial Guinea.
Jammeh's regime was accused by several NGOs of enforced disappearances and harassment of the press and human rights defenders, -- charges it regularly rejected.
Swiss courts have, since 2011, been able to hear serious crimes that have allegedly taken place outside the country, provided that the accused is in Switzerland and cannot be extradited or handed over to an international criminal court.