Nigerian forces fought what they said was a gun battle with Islamist insurgents in the capital Abuja on Friday, but statements from witnesses and the US embassy suggested those involved were squatters.
Nigeria's main intelligence agency said Boko Haram fighters opened fire on security agents in the capital who were searching for a weapons cache in a building under construction.
If confirmed, it would be the first violence linked to the Islamist group to hit the Nigerian capital in months.
However, a security message from the US embassy about what appeared to be the same incident suggested those involved may in fact have been squatters.
The embassy, naming the neighbourhood where the incident occurred, warned of "the potential for civil disturbances in and around the Apo area, Wumba District, Abuja, throughout tonight and over the weekend".
It added that "a violent incident in the early hours of September 20th involving squatters in Apo may be the trigger for such disturbances."
A US embassy spokeswoman declined further comment and Nigerian intelligence agency spokeswoman Marilyn Ogar did not answer calls.
Several witnesses injured in the shoot-out told AFP in an Abuja hospital that eight people were killed, describing the victims as unarmed squatters.
A man who claimed to be at the building at the time of the incident and whose name AFP has withheld for his protection said the owner had come on Wednesday and told them they had a week to leave.
He said the dozens of people living in the house were unarmed, and were not Boko Haram members.
"They give us one week to leave the house, but two days later they come as we sleep," said the man from his hospital bed, where he was being treated for what appeared to be gunshot wounds to his legs.
"We are not staying in the house free. We pay to the guard 200 naira ($1.25, .92 euro cents) per week."
Earlier, Ogar said that security agents began "digging for arms" shortly after midnight based on a tipoff from arrested insurgents.
They then "came under heavy gunfire attack by... Boko Haram elements within the area, which prompted immediate response from the security team," she said in a statement.
Several people were injured in the shootout and 12 were later arrested, according to Ogar.
Police in Abuja told AFP that a number of people were killed, but declined to provide figures.
Nigerian security forces have been accused of major abuses, including killing civilians and arbitrary arrests, though it has consistently denied the allegations.
The incident came as the military sent troops to a town in the country's remote northeast where the Islamists slaughtered at least 87 people earlier in the week.
The military launched a major offensive against Boko Haram four months ago in the northeast.
Since the offensive began, most of Boko Haram's attacks have been concentrated in remote parts of the northeast, typically targeting vulnerable civilians.
Past attacks in Abuja, in the country's centre, have included the August 2011 bombing of a UN building that killed 25 people as well as bombings at newspaper offices and a shopping centre.
This week also saw the deadliest attack since the military offensive was launched.
Late Tuesday, in the remote northeastern town of Benisheik in Borno state, heavily armed Islamists disguised in military uniforms burnt scores of homes and buildings and killed dozens.
They stormed the area in some 20 trucks and were equipped with "anti-aircraft guns" according to a security source who requested anonymity.
Witnesses and army spokesman Ibrahim Attahiru said the security forces were overwhelmed by the Boko Haram assault, and some reported that military personnel abandoned the town amid the killing.
Survivors described a roadside littered with corpses.
Saidu Yakubu of Borno's Environmental Protection Agency told journalists who visited the town Thursday that 87 bodies had been discovered after the massacre, but that toll could rise.
Army General Mohammed Yusuf told journalists who visited the area that a "full reinforcement" of Benisheik would take place.
Tolls earlier this year said the Boko Haram conflict has cost more than 3,600 lives, but the current figure is likely much higher.