Probation offices in Derry have been damaged in an overnight explosion.
British army bomb disposal experts are examining a second device near the scene of the first blast in Crawford Square near the city centre last night.
No injuries have been reported, police said. Dissident republicans opposed to the peace process have been linked to similar attacks in the past.
Northern Ireland justice minister David Ford said: "Whoever planned, assembled and set off the bomb in Derry on Monday night have nothing positive to offer the community. It put the lives of local people at risk and forced them to leave their homes.
“Reports of a second device clearly show the intent to cause injury.
“Damage was also caused to local businesses, including the Probation Service, who provide an invaluable service protecting the community through managing offenders and helping them reintegrate into the community.”
Residents in Crawford Square were evacuated late at night.
A Police Service of Northern Ireland (PSNI) spokesman said: "Crawford Square in Derry remains closed to traffic this morning while police and army technical officers investigate an explosion in the area last night. Northland Road is open to traffic."
Sinn Féin Stormont Assembly member Raymond McCartney condemned those behind the attack.
“Thankfully no one was injured as a result of this reckless attack in a built-up residential area.
“This has brought nothing but disruption to the lives of local people and to the city generally.
“This is a city moving forward and the people have made it clear that they do not want these type of incidents.
“We will not allow a tiny minority to drag us back to the past.”
PA