A bomb left outside the gates of a primary school in Belfast was "significant in terms of its shape", the police have said.
The device was found by a passing police patrol near Holy Cross Boys' Primary School in Ardoyne in the early hours of Sunday.
Pupils returned to Holy Cross on Monday after the Easter break.
The school's principal said he was "shocked and horrified" the device had been left so close to the school.
"There could have been loss of life here," Kevin McAreavey told BBC News NI as children passed the spot where the bomb was found.
"A school should be a safe and secure environment for children."
Staff at the school were trying to keep the day running as normal, he added.
Ch Supt Chris Noble said police officers on foot patrol were most likely the target.
"Whatever fragments of these devices are projected, they're going at some speed and some impact, so that could easily have penetrated a window and there are a number of very vulnerable people living nearby."
He added: "There's very few people, thankfully, who have even seen that device, let alone know what it is, so we want to keep some degree of discretion around the detail of it.
"These individuals are becoming increasingly desperate and increasingly reckless," he added. "There's a blood lust about what they're trying to do."
About 20 residents had to leave their homes during the security operation and were given shelter in a nearby community centre.
Fr Gary Donegan, a former rector of Holy Cross Church who remains involved in community relations in north Belfast, said there was a sense of outrage in the community.
"There's more a sense of unity because people are just appalled by the fact that this would actually happen," he said.
"The thought of that actually happening and the fact that it was a school, the irony is not lost on people."
The chairman of the NI Police Federation, Mark Lindsay, said the device appeared to have been an anti-personnel-type weapon.
"The attackers are terrorising people who live in the area and preventing them from getting on with their everyday lives," he said.
"They didn't care if passers-by or children out playing in the area were caught up in an explosion."