Police have said that a device discovered in Newry, County Down, could easily have detonated.
Officers were called to Ashgrove Road after the suspicious object was found near a house at 15:30 (GMT) on Saturday.
Fifteen families were forced from their homes during the security operation, which lasted a number of hours.
Police said on Sunday that a controlled explosion was carried out and they believe the device was viable.
Most residents have returned to their homes, after some sheltered at the Newry Leisure Centre.
Supt Norman Haslett of the Police Service of Northern Ireland (PSNI) said it was a "reckless act".
"This device could easily have detonated, or have been picked up by anyone, including children. Someone could have suffered serious injury," he said.
"As we continue with our investigation, I want to thank everyone inconvenienced by the alert.
"In particular, I want to thank the 15 families who had to leave their homes as we worked to make the scene safe."
Police said that they are working to establish who was responsible for leaving the device.
Sinn Féin MLA Liz Kimmins described it as a "futile action" and an "attack on the entire community".
"Those responsible for this suspicious object which led to a security alert in Ashgrove Road in Newry have nothing to offer the community.
"Families had to be evacuated from their homes in what must have been a frightening and traumatic experience."
On Saturday, SDLP MLA Justin McNulty said that elderly people and young families had been "forced from their homes into the wind and the rain".
"Those responsible for this device don't care about people in our community," he said.
"People living here don't want this, they don't want to return to the murder, mayhem and tragedy of the past."
Ulster Unionist Party (UUP) councillor David Taylor said it was "totally reckless".
"Their actions have caused significant disturbance and concern to local residents and there is no doubt this device has placed them and the wider community at risk of serious harm," the Newry, Mourne and Down District Council representative said.