Northern Ireland police say the three security alerts they examined in Londonderry on Monday were hoaxes, including one where they carried out a controlled explosion.
The controlled explosion was on a vehicle that was hijacked by masked men in Londonderry, two days after a car bomb exploded in the city on Saturday.
"We can confirm that all three alerts are hoaxes, however, we cannot underestimate the impact these incidents have had on our community," police said.
No one was injured in the blast outside a court on Saturday, but the incident highlighted the threat still posed by militant groups opposed to a 1998 peace deal that largely ended three decades of violence in the British-run province.
The blast came at a time when police in Northern Ireland and European Union member Ireland have warned that a return to a hard border between the two after Brexit, complete with customs and other checks, could be a target for militants.
However, neither Britain's Northern Ireland minister, Karen Bradley, nor local police believe there are any links between Brexit and the incidents in Lononderry, which lies close to the border with Ireland.
"We are not picking up any information that indicates that anybody wants to engage in violence in relation to the Brexit issue, certainly not at this point," assistant chief constable Mark Hamilton told Irish broadcaster RTE.
A fifth man, aged 50, was arrested on Monday in relation to Saturday's attack.
The four other men detained earlier in connection with the incident were released without charge, police said.
There were no details from police on who may have been behind the hijackings in Londonderry, but the main focus of the car bomb investigation is the New IRA - one of a small number of groups opposed to the 1998 Good Friday agreement.
The agreement mostly ended a conflict between mainly Protestant unionists who want Northern Ireland to remain part of the United Kingdom and predominantly Catholic nationalists, who want a united Ireland.
Australian Associated Press