Police are blaming republican dissidents for the bombing of the PSNI training centre in east Belfast just before midnight on Tuesday.
They say an attack was expected.
The device, which contained a few kilograms of explosives, was left at the back gate of the Garnerville college.
Some 100 people at the college were moved, as were residents in surrounding homes.
Little damage was caused and no one was injured, but the attack was politically symbolic.
It was carried out despite the security arrangements at the college where the latest batch of applicants are in training.
Recruitment is now carried out on a 50:50 Catholic/Protestant basis and it is feared that repeated attacks could affect Catholic applications.
The first 44 trainees graduated just under three weeks ago.
The most senior officer in Belfast said: "The first suspicion must be in terms of dissident republican groups but it is at a very early stage."
Mr Alan McQuillan added: "This is not an attack on the police service but an attack on all the people in Northern Ireland who want to move forwards."
Unionist politicians were quick to blame republicans and the DUP said it made little difference whether Provisional or dissident IRA elements were responsible.
The Policing Board, which oversees the development of the new service, and the SDLP said the bombing was an attack on the new policing dispensation.
Prof Des Rea, the chairman of the board, said: "It is disheartening that there are those within the community who wish to stay in the past."
The deputy chairman, Mr Denis Bradley, said the incident "would not halt the peace process".
The Deputy First Minister, Mr Mark Durkan, said the attack was designed to undermine what he called "the new beginning to policing".
"I know that it will not succeed," he said.
"Just as the new police trainees showed courage in joining, they are showing courage now by not letting this attack divert them from their chosen career."
The North's security minister also said the incident would "not deflect the move towards a better future for policing and for Northern Ireland".
East Belfast MP Mr Peter Robinson said the bombing was typical of the Provisional IRA, but he added: "though undoubtedly one of their sister organisations will be asked to take the blame".
He criticised the Sinn Féin president, alleging Mr Gerry Adams had called on republicans to treat the new police service in exactly the same way as they had treated the RUC.