At least 21 detainees have been killed during an attempted escape from Nigeria's secret police headquarters in Abuja, with unconfirmed reports that Boko Haram fighters were involved.
The security breach at the facility in the capital on Sunday raised fresh questions about the government’s ability to tackle the armed group.
However, there was no official confirmation of the involvement of the group, which has been waging an increasingly violent insurgency in Nigeria's northeast this year.
Marilyn Ogar, spokeswoman for the Department of State Services (DSS), which is Nigeria's domestic intelligence agency, said only that detainees had died during the attempt.
"It has been established that 18 casualties were recorded, while two service personnel sustained serious injuries," she said in an emailed statement earlier on Sunday.
"Three other detainees who sustained gunshot injuries during the incident have also died. Meanwhile, (an) investigation is still ongoing to ascertain the immediate and remote causes."
Fears of outside help
Ogar had said earlier that the incident happened at 7:15am (06:15 GMT), when one of its officers went to give the detainees food.
"One of the suspects attempted to disarm him by hitting him at the back of his head with his handcuff," she said.
"His attempt to escape drew the attention of other guards at the facility who fired some shots to warn and deter others."
No details were provided on the number of inmates involved or the reason for their detention. There was no explanation either about how exactly the 21 died.
But Ogar indicated they acted in case the detainees had outside help, as roads leading to the compound were cordoned off "to prevent possible assistance from external collaborators".
The DSS headquarters are near the presidential villa and nearby residents reported hearing gunfire on Sunday morning.
President Goodluck Jonathan's spokesman, Reuben Abati, downplayed the incident, writing on Twitter that the head of state's residence was not affected and there was "no cause for alarm".
"What happened at the SSS HQs has nothing to do with the (Presidential) Villa. President Jonathan is safe & well. Thank you for your concern & support," he added.
The government has previously been criticised for relying too heavily on conventional military means to tackle an enemy fighting a guerrilla war.
Many users of social media initially believed the presidency was under attack, given its proximity to the intelligence agency.