When heavily armed police heard shots ring out from inside Sydney's Lindt cafe, they had no option but to storm in, experts say.
In the wake of the dramatic and tragic siege in the CBD this week, the police handling of the event is being scrutinised.
Sydney Barrister and mother-of-three Katrina Dawson and Lindt cafe manager Tori Johnson both died following the end of the siege, which came after police stormed the Martin Place cafe amid gunfire in the early hours of Tuesday morning.
The self-styled sheik and gunman Man Haron Monis, was killed at the scene.
Questions have surfaced over the tactics used by police to end the siege and whether it could have been brought to a close sooner.
However, experts say officers were right to negotiate with the gunman.
Former counter-terrorism negotiator Belinda Neil said the fact police heard shots ring out from inside the siege indicated an "imminent threat" to the hostages' safety and lives.
"I believe there was also footage of a sniper saying `hostage down'," she told AAP on Wednesday.
"So you have your imminent threat to the lives and safety of the hostages."
So-called `tactical resolution' is a last resort and only ever used if there's an imminent threat to hostages, she added.
Ms Neil, who has written a book on her experience in the NSW police force and with post-traumatic stress, said one never goes into a negotiation trying to resolve it in the first half hour.
"You are dealing with a very intense, volatile situation. Your hostage-taker may be very agitated and angry," she said.
"Your number one priority is to calm the situation and find out what is going on."
The first stage of a negotiation is to calmly build a rapport with the hostage-taker before looking at what their demands and avenues for a peaceful resolution, added Ms Neil, a former homicide detective.
Ms Neill's theory of protracted and complex negotiations is supported by a former senior police office who is familiar with counter terrorism.
"There is a very strong history that negotiations work and time does solve problems," he told NewsCorp.
"The intention would have been to negotiate with him, to try to influence him.
"The longer it goes, the more chance you have got. There is decades of evidence that is the best way."
Asked whether officers could have gained entry to the building via the roof, the officer said it all depended on access. Most likely, police had a contingency plan to enter covertly, either through fire exit or back door.
Sharp-shooters were in place in the Seven Network building across from the Lindt cafe as the siege unfolded.
Former NSW Police Assistant Commissioner Clive Small said there was good reason police didn't try to shoot the gunman through the window.
In the early stages of the siege when Monis was spotted through the glass, it was unclear whether he had a backpack with explosions, how many hostages were inside and or how thick the glass was.
"They were still getting their facts together and working out what the best strategy was," he said.
The gunman also made demands through his hostages, including one to talk to Prime Minister Tony Abbott on the phone.
But Mr Small said that could have ended in Mr Abbott inadvertently saying the wrong thing and setting the gunman off.
Mr Small pointed out Monis didn't have an escape route.
"So he made his mind up that he was going to die there that day."