Elderly residents were forced from their homes during the night following the discovery of a pipe bomb in Banbridge.
The discovery was made in the Granville Gardens area of the town.
Detectives investigating the incident are appealing for witnesses and information.
Police received a report just before 11:05pm that a pipe bomb had been thrown at a house in Granville Gardens, but that it did not ignite.
Police and ATO attended the scene and, following examination, the device was declared viable. It will now be subject to further forensic examination.
Detective Sergeant Stewart said: “An investigation is underway to establish who is responsible for this reckless act and a motive.
“We are appealing to anyone who was in the area at around 11pm and witnessed any suspicious activity, or has information about this incident to contact our detectives on 11, and quote reference 2224 of 09/08/21.”
Alternatively, you can submit a report online using our non-emergency reporting form via http://www.psni.police.uk/makeareport/
You can also contact Crimestoppers anonymously on 0800 555 111 or online at http://crimestoppers-uk.org/
Commenting on the discovery the device, Carla Lockhart MP, said: “Those who left this device have nothing to offer our community.
“They seek to place the local community in danger and I am thankful this morning that there are no reports of injuries or serious harm.
“This is not the first time in recent months the local community has been subjected to this type of activity and it is time for the authorities to act.
The DUP representative added: “Such recklessness and disregard for the safety of local residents is an insight into the minds of those involved in this incident.
“I would appeal for anyone with information in relation to this matter to contact the PSNI. You can do so anonymously via the confidential hotline.
“We need to bring these people to justice, and rid our society of those who place people in danger. Granville Gardens is a quiet area and people just want to get on about with their lives. I will be speaking with Police and other authorities this morning.”