Detectives were granted a 48-hour extension on Friday evening by a judge.
Mr Adams, 65, denies allegations that he was involved in the abduction and murder of the mother-of-ten.
He has been held for questioning since voluntarily presenting himself at Antrim police station on Wednesday.
Mr Adams is the former MP for West Belfast and is currently an elected representative for County Louth in the Republic of Ireland.
An initial 48-hour deadline to either charge or release him was due to expire at 20:00 BST on Friday, but the police successfully applied for more time to question him.
Mr Adams appeared in court via video link, as his lawyers contested the police's application for an extension under the Terrorism Act 2000.
Detectives could have asked for five more days to question Mr Adams, but instead applied to hold him until Sunday evening.
Earlier on Friday, his party colleague Martin McGuinness told a news conference: "Yesterday, I said that the timing of the arrest of Gerry Adams was politically motivated.
"Today's decision by the PSNI to seek an extension confirms me in my view."
Mr McGuinness, who is Northern Ireland deputy first minister, said the detention of Mr Adams was "a very, very serious situation".
He said Sinn Féin supported the progressive elements within the PSNI.
However, he added: "There is a cabal within the PSNI who have a different agenda, a negative and destructive agenda to both the peace process and to Sinn Féin."
He said Sinn Féin had been told this by "very senior members of the PSNI" who had coined the phrase the 'dark side'.
"Am I angry? Yes I am, but it's a very controlled anger," he said.
In regards to Sinn Féin's support for policing in Northern Ireland, he said that would continue if the situation with Mr Adams is resolved in a satisfactory manner.
"If it doesn't, we will have to review that situation."
When pressed if this meant Sinn Féin would withdraw support for the police if Mr Adams was charged, he said: "We're not taking any decision at this time, about anything."
Mr McGuinness added: "I believe Gerry Adams will be totally and absolutely exonerated."
Northern Ireland Justice Minister David Ford of the Alliance Party said: "If there are dark forces within policing, I can see no sign of dark forces."
Mr Ford, who was first appointed minister when policing and justice powers were devolved to Stormont in 2010, said: "I see a police force with very high levels of confidence, higher than the Garda Síochána (Irish police) or many forces in Great Britain.
"I see a police force carrying out its duties properly and appropriately, following up evidential opportunities where they present themselves and operating in conjunction with the community across a range of issues."
Ulster Unionist Party leader Mike Nesbitt said: "It is one thing to scrutinise the police - it is totally unacceptable for their support for police to be conditional on getting political policing in relation to republicans, which Martin McGuinness seems to be suggesting."
Dolores Kelly of the nationalist SDLP said: "For Sinn Féin to threaten to withdraw support from the PSNI because an investigation is not going the way they want it completely unacceptable."
On Thursday, Northern Ireland First Minister and DUP leader Peter Robinson said: "I would suggest to you that it would be political policing if the PSNI had not questioned those that were deemed to have been involved in any way."
Mrs McConville, a 37-year-old widow and mother of 10, was abducted and shot by the IRA. Her body was recovered from a beach in County Louth in 2003.
She is one of Northern Ireland's Disappeared, those who were abducted, murdered and secretly buried by republicans during the Troubles.
She was kidnapped in front of her children after being wrongly accused of being an informer - a claim that was dismissed after an official investigation by the Northern Ireland Police Ombudsman.
The widow was held at one or more houses before being shot and buried in secret.
The IRA admitted in 1999 that it murdered and buried at secret locations nine of the Disappeared.
Sixteen people are listed as "disappeared" by the Independent Commission for the Location of Victims' Remains, which was established in 1999 by a treaty between the British and Irish governments.
Despite extensive searches, the remains of seven of them have not been found.
Last month, Ivor Bell, 77, a leader in the Provisional IRA in the 1970s, was charged with aiding and abetting the murder.
There have also been a number of other arrests over the murder recently.
Police in Northern Ireland have been given more time to question Sinn Féin leader Gerry Adams in connection with the 1972 murder of Jean McConville.