Dissident republicans are being blamed for several suspected firebomb attacks in County Down.
Homes had to be evacuated during the fires
Two stores were destroyed and two others badly damaged in a series of overnight fires in Newry.
Police said it was a "devastating attack" and warned there could be unexploded devices at other premises.
Sinn Fein blamed republican "micro organisations". Ulster Unionist Danny Kennedy said dissidents were "marking the anniversary of internment".
About 125 firefighters and 26 appliances tackled the blazes which erupted at about 0225 BST on Wednesday.
They destroyed JJB Sports and CarpetRight stores, whilst a TK Maxx store and MFI outlet were badly damaged. Damage has been put at hundreds of thousands of pounds.
About 20 homes were evacuated during the incident.
Chief Inspector Gary Hagan said a link to dissidents was one line of inquiry.
"It is difficult to comprehend or understand what would be in the mind of people that would want to carry out these kind of attacks, that have left potentially large numbers unemployed and the commercial heart... badly, badly damaged," he said.
He also urged the business community to check their premises.
"It is possible that some of these devices are still in location and haven't actually activated. It is vitally important that we locate those as soon as possible," he said.
"This has happened in Newry last night and it is possible it could happen in other cities, in other towns in other times."
Andy Gilkinson, manager of the CarpetRight store, said he feared some staff members may lose their jobs.
"It's just devastation. I thought the days of this were all gone, that it was done and dusted but what can you say? The place is just ruined," he said.
"I'm sure I'm speaking on behalf of CarpetRight and the head office across the water, I'm sure they will rebuild again."
Assembly member Danny Kennedy said the attacks had struck at the heart of the city's commercial centre.
"It is absolutely sickening that these people have terrorised the commercial centre of Newry with an action intended to strike fear into the hearts of businesses and shoppers alike," he said.
Davy Hyland of Sinn Fein said the attacks looked like the work of dissident republicans.
He added: "There is very, very little if any support for what happened. Newry has been through difficult times, but in recent years it has gone through a resurgence. Jobs are now in danger of being lost."
A JJB Sports store was destroyed in the blaze
David Hanna of the Chamber of Commerce said the apparent return of firebomb attacks was "distressing".
Walter Johnston of the Fire and Rescue Service said it was "most unusual" to have this type of incident at the same time involving four premises.
"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," 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."
A Northern Ireland Office spokesman said: "This kind of incident is an attack on the whole community.
"Attacks such as this are a matter of serious concern. It places both jobs and livelihoods in jeopardy."
Meanwhile, Army technical officers have been dealing with a number of security alerts in Newry, County Down.
Police closed the Newry bypass in both directions at 1800 BST on Wednesday because of a security alert on the Dublin Road.
Firefighters have tackled four fires in Newry
An alert at Merchant's Quay has ended after it was declared a hoax, as was another alert at Bridge Street. Hill Street was also searched, but nothing found.
Wednesday is the 35th anniversary of the introduction of internment when hundreds of men were arrested in nationalist areas and detained without being charged.
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