There are mounting fears over the fate of a man who went missing 11 days ago after tweeting pictures of an attempted jail break in Abuja, Amnesty International said today as joined a twitter action on his behalf.

The man, identified by family members as Yusuf Siyaka Onimisi, has not been seen or heard from since 30 March after tweeting developments and images of fighting between Nigerian security forces and detainees trying to escape from the State Security Service (SSS) headquarters. He was reportedly working at an electrical sub-station within the nearby presidential compound when the fighting, which left 21 detainees dead, erupted.

“Nigerian security forces should immediately disclose the whereabouts and legal status of Yusuf Siyaka Onimisi. If he is in detention, Nigerian authorities must either charge him with a recognizably criminal offence or release him immediately,” said Netsanet Belay, Research and Advocacy Director for Africa at Amnesty International.

Yusuf Siyaka Onimisi’s whereabouts are still unknown but witnesses reported that he was arrested by plainclothes armed men outside his office, raising fears the authorities have subjected him to an enforced disappearance, putting him at increased risk of torture or extrajudicial execution. The SSS has so far neither confirmed nor denied that they are detaining him.

Amnesty International spoke to Sanusi Onimisi, the missing man’s brother, who said Yusuf is the eldest of five siblings and had been working as a qualified electrical engineer.

"He has been arrested for 11 days without any information being given to the family. If the government treats people like this, who are the terrorists? Who is the threat? The family has been kept in darkness. If you are holding him or if there is to be a sentence of death, then tell the family,” said Sanusi Onimisi.

Amnesty International has joined a twitter action launched by local activists, using the hashtag #FreeCiaxon, calling on the SSS to release Yusuf Siyaka Onimisi.

The SSS has launched an investigation into the attempted jailbreak in Abuja and alleges that the detainees were suspected Boko Haram members. However, the government has not yet given a full account of the incident.

In a report released in late March, the organization documented the Nigerian security forces’ alleged extrajudicial executions of more than 600 escaped detainees from Giwa Barracks, a military detention centre in Maiduguri.