A Kiwi mother says her 4-year-old son has been left traumatised after they became caught up in the Las Vegas mass shooting.

The mother, who only wants to be identified as Michelle, was at the Route 91 Harvest festival with her husband, son, and her 17-year-old brother, when gunman Stephen Paddock sprayed bullets into the crowd, killing at least 59 people.

The Kiwi mum told Fairfax the "horrific" ordeal left her son traumatised.

Agents from the FBI continue to process evidence at the scene of a mass shooting. Photo / AP
Agents from the FBI continue to process evidence at the scene of a mass shooting. Photo / AP

"My son said to me 'mum, why was he trying to kill us? All we were doing was watching music'."

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Michelle told Fairfax she was screaming and telling her son and husband she loved them as the gunfire "just kept going".

The Kiwi family were watching the concert from near the back of the crowd when gunshots started ringing out.

Michelle thought the sound was fireworks until concertgoers started running. It was then she realised someone was firing bullets at the crowd.

"We didn't know whether the shooter was in the venue, where he was firing from, or whether he was coming in our direction," she told Fairfax.

"I remember running and then falling over. My husband was carrying our son and picked me up by my arm as well and got us into a vendor stall with a lot of others."

The group huddled at the stall until the gunfire stopped, at which point they ran again, making it behind some bleachers just before the shooting resumed.

In the midst of the madness, Michelle's brother became separated from the trio.

After making their way to a casino across the road, her husband tried contacting her brother but there was no answer.

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The family was luckily reunited at their hotel room later on.

"It seemed like an eternity before my brother got there," Michelle said.

"When he walked through the door all the tears came out and we just cried and cried. There was very little sleep that night."

While the family left Las Vegas the day after the shooting, the traumatic incident remained etched in their memories, particularly that of Michelle's 4-year-old son.

"It breaks my heart that he had to go through it. He knows what happened and has been asking questions, and we're answering them the best we can."

Michelle and her family luckily escaped uninjured apart from a few bruises and scrapes.

Las Vegas gunman Stephen Paddock planned to live, 'plotted more attacks'

Investigators are looking into the possibility the Las Vegas shooter planned more attacks, including a car bombing, says a US official.

The official stressed the information was preliminary and investigators haven't uncovered any solid evidence yet.

The official, who was briefed by federal law enforcement officials, wasn't authorised to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.

Clark County Sheriff Joe Lombardo says it appeared Paddock had planned to survive Sunday's attack and had an escape plan.

Lombardo declined to reveal why he believed that Paddock had an exit planned, but said he was confident in the claim - and said that his scheme was so elaborate that he would have to be a "super hero" to pull it off alone.

Several law enforcement sources told NBC that investigators are looking to speak with a woman who was seen with Paddock in his last days, when his girlfriend was out of the country. They don't know if she's connected to the attack but want to speak with her to fill in their timeline of the lead-up to the attack.

Lombardo said Paddock lived a "secret life" and "meticulously planned on the worst domestic attack in United States history".

"Stephen Paddock is a man who spent decades acquiring weapons and ammo and lived a secret life, much of which will never be fully understood," Lombardo said.

He also announced that the count of people injured in the attack was lower than initially stated - 489, not 527 - due to confusion at hospitals.