Peter Biar Ajak was arrested after saying that Kiir and Machar should resign to and allow a new generation to lead.

On July 28, Deng Ajak-Agutdau received a phone call with news that his brother had been arrested at Juba airport by the National Security Services (NSS), South Sudan’s intelligence agency.

Peter Biar Ajak had been planning to travel to a youth forum he had organised in Aweil.

He was one of at least 20,000 children who were dubbed the “lost boys” of Sudan after they were separated from their families during the country’s second civil war, which ended in 2005.

While many were displaced to refugee camps across East Africa, some were resettled in the United States in the early 2000s, including Ajak.

He went on to study at La Salle, Harvard and Cambridge. He has worked as an economist for the World Bank, founded the Centre for Strategic Analysis and Research (C-SAR) in Juba and was the chairman for the South Sudan Youth Leaders Forum’s (SSYLF).

“Peter has always spoken out about social justice in South Sudan and has been genuine about seeing a better future for the country,” Ajak-Agutdau told Al Jazeera.

By the time of publishing, officials of the government of South Sudan had not responded to several requests from Al Jazeera seeking comment.

Peter has been critical of the peace efforts. There has been no official communication from the government about his arrest but talk on social media is that the NSS wants to investigate his activism. Deng Ajak-Agutdau, Peter Ajak's brother

Separating from its northern neighbour, South Sudan was formed in 2011 after several decades of conflict.

However, the new country fell into civil war two years later.

On August 5, President Salva Kiir and his rival, rebel leader Riek Machar, reached a power-sharing deal to end the fighting. The new agreement is the second attempt at peace since a 2015 deal fell apart.

“Peter has been critical of the peace efforts,” said Ajak-Agutdau. “There has been no official communication from the government about his arrest but [talk] on social media is that the NSS wants to investigate his activism.”

Ajak-Agutdau speculated that this might include Ajak’s involvement with the SSYLF and his appearances on regional media outlets.

In July, Ajak tweeted his appearance on Kenya’s NTV show, AM Live, arguing that both Kiir and Machar should exit the government and allow a new generation to lead South Sudan.

He said trying the same methods after repeated failure did not make sense, and would inevitably lead to more fighting.

Hundreds ‘arbitrarily detained’

A Human Rights Council Commission of Inquiry report in March on incidents between 2016 and 2017 warned of arbitrary detention.

“Hundreds of South Sudanese have been arbitrarily detained in utterly deplorable conditions by the [NSS] since the conflict began in 2013, including government critics, activists and journalists”, said Seif Magango, the deputy regional director for East Africa, the Horn and the Great Lakes at Amnesty International, which has been documenting arbitrary detentions in the country.

Numbers are unclear but one organisation – the South Sudan Human Rights Observatory (SSHRO) – has indicated that thousands could be affected.

Ajak-Agutdau says that family members had spoken to Ajak and seen him at the “Blue House”, a notorious prison inside NSS headquarters, where detainees are allegedly tortured.

They told him he appeared unharmed.

“In South Sudan, the [NSS] is a very large institution … it’s very repressive and they’re very well-known for harassing people,” said Jehanne Henry, associate director for Africa at Human Rights Watch.

She told Al Jazeera that this was “part of its strategy to keeping all critics and challengers at bay to remain in power. The government also holds civilians in military detentions at various locations, which is also unlawful under human rights law”.

This disregard for fundamental human rights must come to an end. South Sudanese authorities must release, or charge all those still arbitrarily detained with a recognisable offence. Seif Magango, deputy regional director at Amnesty International

There have been international calls for the NSS to free Ajak in a campaign organised by his friends and colleagues including Aaron Spence, a digital and analytics manager in Philadelphia and fellow La Salle graduate.

Among those who have spoken out are Majak D’Agoot, former Sudan deputy intelligence chief and a former political detainee.

Senators Chris Coons (Delaware) and Cory Booker (New Jersey) have also called for his immediate release, saying that punishing critical voices undermined the peace process.

Nearly 500 people have signed an open letter supporting Ajak’s release, while close to 7,000 people have supported a petition.

“We are fortunate that Peter had a wide network of people who cared for him and supported him for his endeavours over the years and we’re able to stand up. Not a lot of people on the ground have that,” Spence told Al Jazeera.

“So, what we want is yes, the swift unconditional release of our friend but we also want to use our voice to amplify them to show that there are other people being detained in this way in such a cruel location.”

Other people who are currently imprisoned include Dong Samuel Luak, a South Sudanese human rights lawyer and Aggrey Idri, a vocal government critic from the opposition.

Both were removed from Nairobi, Kenya, and taken to the NSS headquarters before they disappeared on January 27, 2017.

Mike Tyson, Alison Mogga Tadeo, Richard Otti and Andria Baambe were held without charge in 2014 for alleged links with the opposition but died in detention in 2017.

Adil Faris Mayat, director of the South Sudan Broadcasting Corporation, was held without charge in July 2017 after not broadcasting President Kiir’s Independence Day speech, but was released shortly after appeals from domestic and international rights groups.

Critics have said there is urgent need for reform, including of the 2014 National Security Bill of South Sudan, which props up NSS activities.

“This disregard for fundamental human rights must come to an end,” says Amnesty’s Magango. “South Sudanese authorities must release, or charge all those still arbitrarily detained with a recognisable offence, and reduce the overly broad powers of the [NSS] that in effect give it carte blanche to arrest and detain people without justifiable cause.”