Fires which swept through four business premises in Newry were probably started by incendiary devices, the Fire and Rescue Service has said.
Firefighters have tackled four fires in Newry
About 125 firefighters tackled blazes which erupted at about 0215 BST.
JJB Sports and Carpetrite have been destroyed, whilst TX Maxx and MFI have been badly damaged. About 20 homes were evacuated during the incident.
Meanwhile, Army technical officers are at the scene of a number of security alerts in Newry.
The tax office at Merchant's Quay and the jobs benefits office at Bridge Street have been evacuated.
David Hanna of the Chamber of Commerce said the apparent return of firebomb attacks was "distressing".
The fires affected businesses on Merchant's Quay, Monaghan Street and the Damolly Retail Park.
Walter Johnston of the Fire and Rescue Service said: "It is most unusual to have this type of incident at the same time involving four premises.
"As soon as the buildings cool down we'll be going through them and the police will be helping us.
"We're almost certain it was malicious fires, but how they actually were started we are not sure yet. The buildings were totally secured when the fire crews arrived."
There are a number of security alerts in Newry
The police said a forensic examination of the scene would take place throughout the day, and the fires were being treated as suspicious.
They are asking keyholders of major businesses in the area to report anything suspicious.
Sinn Fein MP Conor Murphy blamed "republican micro organisations".
"The destruction of these businesses has caused much anger in the Newry area this morning and are particularly worrying for those people employed in these four stores," he said.
Ulster Unionist Danny Kennedy said the attacks struck at the heart of the city's commercial centre.
"They are intended to strike fear into the many shoppers. It really is completely sickening," he said.
SDLP assembly member Dominic Bradley described the attacks as "futile".
Mr Bradley said: "This type of wanton destruction of property and jobs only underlines once again the utter futility of violence and should be condemned without reservation by all right thinking people."
About 20 families in the Lisdrum area had to moved out of their homes.
Meanwhile, a security alert along the Newry railway line near Cloughoge Bridge has ended.
Translink said trains had not been affected.
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