Gambia's former spymaster and head of the National Intelligence Agency (NIA), Yankuba Badjie, has been arrested.
Rights groups say the NIA was involved in the disappearance and torture of critics of ex-President Yahya Jammeh.
Mr Badjie is the first of Mr Jammeh's security officials to be taken into custody by the new government.
Mr Jammeh was forced to resign after 22 years in power by The Gambia's neighbours last month after refusing to accept defeat in December's elections.
He has now gone into exile in Equatorial Guinea.
Mr Badjie was arrested on Monday along with another former NIA employee.
The agency is cited by Human Rights Watch (HRW) as being an instrument of serious abuses under Mr Jammeh.
In a 2015 report, HRW painted a bleak picture of human rights in The Gambia:
"The government of President Yahya Jammeh frequently committed serious human rights violations including arbitrary detention, enforced disappearance, and torture against those who voiced opposition to the government," HRW said.
"State security forces most frequently implicated in violations were members of the National Intelligence Agency (NIA)."
According to HRW, those targeted by the intelligence agency included journalists, political opponents, and lesbian, gay, bisexual, and transgender (LGBT) people, many of whom fled The Gambia out of fear.
'Dead or alive'
No official reasons were given for Mr Badjie's arrest.
But he was taken into custody after new President Adama Barrow promised to reform the NIA, which he has renamed the State Intelligence Service.
Mr Badjie is alleged to have personally presided over torture sessions of government critics at NIA chambers.
He has not commented on the various allegations rights groups have made against him.
Human rights activist Lamin K Saidy who is based in Banjul hailed Mr Badjie's arrest.
"We are hoping that lots of revelations [will be made]," Mr Saidy told the BBC.
"A lot of families have not seen their loved ones who were picked up by the notorious NIA."
"We are hoping that having [Badjie] in custody will allow those families to have answers as to where their loved ones are - whether they are dead or alive."