(CNN) Four men arrested on Sunday in connection with Saturday's car bombing in Northern Ireland have been released "unconditionally," police said.
A 50-year-old man arrested Monday following the incident in Londonderry remains in custody.
No one was injured in the car bombing, one of a series of alleged violent incidents across Northern Ireland in recent days, amid concerns for the country's hard won peace raised by ongoing uncertainty over Brexit.
Derry was on high alert Monday, as police responded to a number of calls they later said were hoaxes.
In one incident, a white transit van was reportedly hijacked by three masked men who "threw an object in the back before abandoning it" around midday. Nearby homes were evacuated, after which police carried out a controlled explosion.
Just over two hours later, police dealt with a second security alert in Derry, after a delivery driver's vehicle was hijacked by four masked men. "One of them reported to have had a gun," police said.
Hours later, a third incident in the city was confirmed by police, who said there was an "attempted hijacking of a local bus service in the area of Moss Road."
Meanwhile, in the capital Belfast, roads were closed around Springfield and Lanark Way near a peace wall in the north of the city, due to the "discovery of a suspicious object."
Police in Derry later said three reports in the city had been confirmed to be "hoaxes."
"However, we cannot underestimate the impact these incidents have had on our community," said Derry District Commander Superintendent Gordon McCalmont.
More information is emerging about the car bombing that shook central Derry on Saturday night.
According to police, at least two armed men hijacked a pizza delivery car and installed a bomb inside, which was then driven to the Bishop Street Courthouse before it was detonated.
In CCTV footage released by police, the car can be seen stopped before the driver jumps out and runs away. Shortly before the explosion, a group of young people can be seen walking past the vehicle, narrowly avoiding being caught in the blast.
Assistant Chief Constable Mark Hamilton told reporters the "main line of inquiry is against the New IRA." The group is a dissident offshoot of the Provisional Irish Republican Army (IRA), which agreed to a ceasefire in 1997 after over three decades of violence and supported peace talks with the UK.
"The New IRA, like most dissident Republican groups in Northern Ireland, are small, largely unrepresentative and just determined to drive people back to somewhere they don't want to be," Hamilton said.
He described the bombing as "a callous act, a deliberate act against the people of Derry" and a "deliberate attempt to harm this community."
No group has yet claimed responsibility for the car bomb.
Local Gina McFeely said she was in her flat, getting ready to go to her nephew's 21st birthday party, when she heard a loud bang.
"Everyone in the neighborhood came out into the street," she said. "Then I saw the car in flames, flames going up, and I said, 'Oh my God!' It's been a long time since a bomb went out in Derry. It was a shock."
Despite the explosion, she went ahead to the party.
"It doesn't make me afraid -- I am from Derry, we have lived though this before -- but it makes me angry," McFeely said. "We don't want to go back to this."
Derry mayor John Boyle said he was "absolutely appalled by this terrible act of violence."
"I utterly condemn this attack which could easily have resulted in loss of life or injury," he said on Twitter. "The perpetrators do not speak for the people of Derry and Strabane!"
Simon Coveney, deputy prime minister of the neighboring Republic of Ireland, also condemned the attack, saying "there is no place and no justification possible for such acts of terror, which seek to drag Northern Ireland back to violence and conflict."
The bombing raised fears that sectarian violence might be revived in Northern Ireland, where a tenuous peace has been threatened by the ongoing debate over how the UK will leave the European Union following the 2016 Brexit referendum.
A strong majority in Northern Ireland voted to remain in the EU, and many fear a return to a hard border with the Republic of Ireland -- currently unnecessary because of EU free movement laws -- could spark unrest.
More than 3,500 people died in the decades-long conflict in Northern Ireland -- between Catholic Irish nationalists and Protestants who were pro-British -- known as "the Troubles." The Belfast/Good Friday Agreement of 1998 was a turning point for the region ending years of bloodletting.
In recent years, the two main political parties -- the Democratic Unionist Party and Sinn Fein -- had worked together in a power-sharing executive, but it collapsed in 2017 and despite extensive talks has yet to be restored.
The DUP's representatives in Westminster currently prop up British Prime Minister Theresa May's minority Conservative government, an arrangement many in Northern Ireland condemned for compromising London's objectivity on politics in the country.
The second-largest city in Northern Ireland, Derry was heavily militarized during the Troubles and the site of numerous acts of violence. The name of the city itself is a point of contention for many, with nationalists, who are in favor of a united Ireland, calling it Derry and unionists, who want to remain part of the United Kingdom, calling it Londonderry.