The 45-year-old man suspected of carrying out the Darwin shooting came to the attention of the police twice on Tuesday, hours before the deadly attack.

With police set to charge the suspected gunman later on Thursday, details about the motivation continue to emerge.

The Northern Territory police commissioner Reece Kershaw revealed that the ammunition used during the shooting was purchased earlier on Tuesday by “a well-known associate” of the gunman, and that the shooter may have been in-part motivated by “an issue relating to a female”.

It has also emerged two of the victims were deliberately targeted by the gunman.

One was 52-year-old New Zealand national Robert Courtney, a security guard at a Darwin casino.

A second, Michael Sisois, 57, was shot dead in the carpark of the Buff Club on the Stuart Highway on Tuesday night. It’s understood he was a former colleague of the gunman’s.

Kershaw also revealed that a member of the public called police to an address outside Darwin before the shooting on Tuesday.

Kershaw said police were called to an address in Humpty Doo about 5pm, after reports of a man acting suspiciously and possibly under the influence of drugs. He said by the time officers arrived at 5.20pm the man had left.

Asked by reporters on Thursday whether the man was armed at the time, Kershaw said: “Possibly, yes, there was a sighting of a firearm but we’re working through that.”

It was the second time the man had come to police attention that day.

At about 10.52am police pulled the man over for doing 94km/h in an 80km/h zone on the Arnhem Highway driving a silver Proton.

He said that a review of the officer’s body-worn camera showed “he was not displaying any adverse behaviour. Nothing out of the ordinary was observed by the officer.” Police issued the man a speeding infringement notice but did not search the man’s car.

Asked whether officers should have searched his car given his parole status, Kershaw said there was “no reasonable suspicion” to allow a search.

The gunman was also clocked speeding by two mobile speed cameras at 11.03am and 12.25pm.

Kershaw said he was satisfied with the police response in the lead-up to the shooting.

“Look, my information is he spent quite a long time out there going from address to address,” Kershaw said. “The call that went in around about 5pm … we were dispatched and arrived about 5.20pm. I’m very satisfied with our response of when he was alleged to be in Humpty Doo.”

Kershaw also revealed the ammunition and knife used in the attack were bought legally that day by a third party who was “a well-known associate” of the gunman. Kershaw said it was “not clear yet” whether the third party was an accomplice to the shooting.

On Tuesday the gunman killed four people in a deadly hour-long shooting rampage throughout Darwin. Using a stolen pump-action shotgun, he moved from the Palms Motel in Darwin to a home in the inner-city suburb of the Gardens, the Buff Club and an industrial site in the suburb of Woolner.

The gunman, who has a long criminal history, was on parole after being released from prison in January after serving four years of a six year sentence. He breached his parole in April after missing his 10pm curfew and was detained for two weeks.

It comes as more details emerge about the victims in the deadly shooting. One of the victims was Robert Courtney, a 52-year-old New Zealand national who worked as a security guard at the Darwin casino.

Resort manager Mark Bennett said in a statement Courtney was a “valued member of our security team and we extend out deepest sympathies to his family and friends at this difficult time”.

The gunman’s victim at the Palms Motel was 33-year-old PhD student and taxi driver Hassan Baydoun, who was taking a meal break when he was killed.

Kershaw said police believed Courtney and Sisois were targeted killings, while Baydoun and the 75-year-old man from the Gardens were not known to the gunman.

Multiple witnesses have said that while at the motel, the gunman was yelling out for a man named “Alex”, who police said on Wednesday had been interstate. Asked about the nature of the gunman’s dispute with “Alex”, Kershaw said it may have been related to a woman.

“There may or may not have been an issue relating to a female involved in this, so there was a number of issues that were actually avenues of inquiry that we are following in relation to that,” Kershaw said.

He also said there may have been a drug motivation.