The Continuity IRA is being blamed after a series of bomb alerts brought parts of Belfast to a standstill during yesterday's rush hour.
Anger erupted after separate incidents shut down the motorway at the height of the morning commute and forced 30 families to leave their homes.
A suspicious object – later declared a hoax – led to a seven-mile stretch of the M1 between Belfast and Lisburn being closed in both directions.
In a separate alert, residents at Brae Hill Link in the Ballysillan area of north Belfast were evacuated from their homes following the discovery of a viable pipe bomb.
A viable device was also recovered following a third incident at Old Brewery Lane in west Belfast.
Security sources believe the Continuity IRA was involved in the M1 hoax, as well as a similar incident which brought major disruption to Finaghy last week.
The incidents are not being linked to the two viable devices discovered elsewhere yesterday.
On a day of disruption, the M1 was closed in both directions between the Broadway and Saintfield roundabouts shortly after 3am yesterday morning and did not reopen until 8am.
The alert was sparked by a suspicious object at the Kennedy Way offslip.
It led to massive tailbacks at one of the busiest times of the week, with knock-on congestion affecting other routes into Belfast.
Among the motorists caught up in the chaos was Samuel Morrison.
His commute from Dromore in Co Down to east Belfast normally takes 45 minutes – but yesterday it lasted more than two hours.
"I left home at 7.50am and arrived at work shortly before 10am," he said.
"When I was sitting in the traffic I was wondering whether it was terrorists or criminals who were responsible.
"It was very frustrating.
"It was dead slow the whole way, even when the motorway was reopened because of the backlog.
"Monday morning is bad enough at the best of times, but I wasn't expecting this."
A second motorist, Nuala Adams, said her journey from Lisburn to Stormont, which normally lasts 40 minutes, took more than double that yesterday.
"Lisburn was at a standstill for a big part of the morning," she said.
"It was well over an hour and 20 minutes by the time I arrived at work."
The Belfast Health Trust said the chaos caused delays for some nursing staff. Colleagues remained on duty until they arrived for work.
"There was a slight delay on start times for theatres but this caused minimal impact and no patients were cancelled," said a Trust spokesperson.
"Overall, the quality of care received by patients and clients was not compromised."
Police later said the object which sparked the alert was an elaborate hoax.
Four men were arrested in connection with the incident.
The suspects, two aged 26, one 25 and one 53, were stopped in a car on the Andersonstown Road in west Belfast.
Police said the arrests were made by detectives investigating dissident terrorist activity.
A number of properties in west Belfast were searched as part of the investigation.
It was the second time in five days that a hoax alert has caused misery for commuters.
Last Thursday an alert at Finaghy Halt station in south Belfast saw the rail line closed for more than 24 hours.
Meanwhile, two pipe bombs were found after the security alerts at Brae Hill Link and Old Brewery Lane.
Both areas were closed as Army bomb experts examined the scenes. Detectives are investigating a possible link between the two incidents.
SDLP councillor Tim Attwood condemned the alerts.
"These series of security alerts across Belfast are deeply disruptive for families on the school run and for people going to work," he said.
"The elaborate and cynical hoax at Kennedy Way this morning led to major chaos on the roads and resulted in many people being late for work and missing appointments.
"People in this city deserve to go about their everyday lives without the fear of security alerts and road closures."