The bomb which killed Constable Ronan Kerr was the size of a lunch box and weighed up to half a kilogramme, police said today.
The under-car device was probably detonated using a mercury tilt switch and was housed in a grey plastic container, Detective Superintendent Raymond Murray said.
He believes the device could have been brought into the Omagh, Co Tyrone, housing estate anytime from last Thursday onwards.
The detective said the device included a safety timer but that was probably not the means of detonating it.
"We believe it may be up to 500g of high explosives, that is an initial forensic assessment," he said.
"History and previous attacks of this nature tell us the most likely means of detonation is a tilt switch, however we cannot confirm that and we have not closed our minds to options."
Forensics experts have been combing the residential area for clues left by the explosion. The type of explosive is still under investigation, Mr Murray added.
"It was most likely housed in some sort of thick grey plastic box," he said.
He added: "Something around the size of a lunch box."
Pc Kerr, 25, a Catholic recent recruit to the police service, was getting into his car when the booby trap bomb exploded on Saturday afternoon.
He was on his way to work. He lived with two male friends who were not police officers.
One line of inquiry is a number of dissident threats made in Omagh beforehand. The senior detective did not say which dissident group was responsible.
Examining CCTV security cameras at the development forms a major part of the inquiry. Extensive trawls at the scene have uncovered component parts of the device which are being analysed.
Mr Murray said he was appealing for information about all vehicles which went into the Highfield estate after tea time on Thursday until the time of the blast at around 4pm on Saturday.
Pc Kerr returned home from duty at around 1am on Friday morning.
Mr Murray added: "The bomb planters had to come into the estate and plant the device.
"The movements of people and vehicles in that development in a certain time-frame is absolutely critical to us identifying potential bomb planters."
Police have been conducting door-to-door inquiries but he urged people to come forward without awaiting a knock.
The detective added: "The answer to the question of who planted the bomb and can we catch them lies in that detail.
"Amongst ordinary people living ordinary, decent lives were terrorists."