A US official says investigators believe the Las Vegas shooter may have hired a prostitute in the days before the shooting, and they are interviewing other call girls as they look for clues into his motive.

The official said on Friday that prostitutes are among the hundreds of leads they are pursuing as part of their investigation into Stephen Paddock.

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

They also revealed that Paddock and his girlfriend Marilou Danley visited the Middle East on a cruise sometime during the past few years.

They did not reveal the significance of any of the trips. Officials have repeatedly dismissed ISIS's attempts to claim responsibility for the atrocity.

In Paddock's room, officials found a piece of paper containing a number of phone numbers but they reiterated no suicide note was found.

A US official says investigators believe the Las Vegas shooter, Stephen Paddock, may have hired a prostitute in the days before the shooting. Paddock is seen in this undated file photo

The official said Friday that prostitutes are among the hundreds of leads they are pursuing as part of their investigation into Paddock

A family friend of Paddock's revealed on Friday that he frequently enjoyed the company of prostitutes who were laid on and facilitated by some Vegas casinos.

Australian businessman Adam Le Fevre was in a relationship with the sister of Marilou Danley, Paddock's girlfriend.

Le Fevre told A Current Affair he went to Las Vegas with Paddock, 64, two years ago with his partner Liza and her sister Marilou, and had travelled to the Philippines with him twice, first in 2013.

Madisen Silva (right) and Samantha Werner (left) embrace Friday at a makeshift memorial for victims of the mass shooting

Jamie Lambert visits a makeshift memorial for the victims of the shooting on Friday

A woman places a candle in front of one of many white crosses set up for the victims of the Route 91 Harvest music festival mass shooting

Local and federal officials announced they are erecting billboards around the city appealing to the public to come forward with information about the shooting

A woman places a flower in front of one of many white crosses set up for the victims of the Route 91 Harvest music festival mass shooting in Las Vegas on Friday

He said Paddock was 'condescending' towards Danley, 62, making her 'nervous and jittery', while also making the most of prostitutes offered to him by casinos.

Referring to offers of free prostitutes offered to Paddock by casinos in Las Vegas, Le Fevre said: 'I did have no questions that some of those offers had been accepted, yes'.

He said the relationship between the shooter and his partner was 'not loving' or 'caring'.

'I experienced Steve talking to Marilou in an abrupt manner at times. Marilou... seemed very nervous and jittery around Steve, he would talk in a condescending way at times.'

Le Fervre said he was confident Danley had no knowledge of the planned attack, in which when Paddock opened fire on a music festival from his suite on the 32nd floor of the Mandalay Bay casino before shooting himself dead.

Law enforcement have said they are 'confident' Paddock carried out the Mandalay Bay massacre from his hotel room alone.

Las Vegas Metropolitan Police Department Undersheriff Kevin McMahill said on Friday that investigators are sure no one else was in the room before he shot dead 58 and injured almost 500 on Sunday.

Earlier, NBC News had reported a mysterious charger that did not match any of Paddock's cellphones - raising the possibility that another person had been with Paddock beforehand.

However, later police said that they have now matched all the cell phones and all the chargers and that they belong to Paddock.

At a press conference on Friday, Las Vegas Metropolitan Police Undersheriff Kevin C. McMahill said they were no closer to establishing a motive for the mass murder

'We're very confident there was not another shooter in that room,' said McMahill during a press conference in Las Vegas on Friday afternoon.

'What we cannot confirm is whether anybody else knew about this incident before he carried it out.'

This comes as local and federal officials announced they are erecting billboards around the city appealing to the public to come forward with information about the shooting.

The billboards will have the message 'If you know something, say something' and investigators hope this will help generate leads for the case which has so far left law enforcement struggling to find a motive.

'There are still a number of people out there that know that something looked out of place,' said McMahill

'Someone may have been acting suspiciously that night, or in the years prior, the months prior. Someone that may have seen something or knows something.'

A note containing a series of numbers was found on a nightstand in Paddock's room at the Mandalay Bay hotel after the shooting. Paddock's body, two of his weapons, and ammunition are seen in his hotel room shortly after he perpetrated the massacre

Police confirmed that Paddock was the only gunman, as suspected, and said there was no proof that anyone else accessed the room in the days before the attack

The FBI's special agent in charge in Las Vegas said that the idea is to reach 'as many people as we possibly can' and that they will not stop until 'they have the truth'.

But at the press conference on Friday, McMahill said detectives were no closer to understanding what motivated Paddock's killing spree.

'To date we have run down well more than 1,000 leads in this investigation. Some of it has helped create a better profile into the madness of this suspect but we do not still have a clear motive or reason why.

'In the past, terror attacks or mass murder cases, motive was made very clear by a note that was left by a social media post, by a telephone call that was made.

'Today, in our investigation, we don't have any of that uncovered. I wish we did.'

He confirmed that Paddock was the only gunman, as suspected, and said there was no proof that anyone else accessed the room in the days before the attack.

Authorities are however still investigating whether someone knew Paddock was about to commit the worst mass shooting in US history.

Workers board up a broken window at the Mandalay Bay hotel where Paddock conducted his mass shooting

The only other named person of interest is Paddock's girlfriend, Danley, who was in the Philippines on Sunday night.

Paddock paid for her ticket there and also wired her $100,000 in cash but Danley, 62, believed it was his way of breaking up with her.

She flew back to the US on Wednesday at the request of police after earlier telling family that her conscience was clear.

Detectives appeared convinced on Friday that someone else, somewhere, held information which could solve the mystery of what drove Paddock to commit the killings.

'It's really imperative that the listening public have a very clear understanding that there are still a number of people out there that know something looked out of place.

'If you know something, you need to say something,' McMahill said.

One suggested motive that was put forward by a reporter on Friday was that Paddock was 'obsessed' with the month of October however McMahill could not give any credit to the suggestion that it is what may have caused the killings.

He said they were 'keenly' pursuing his medical records to verify reports that he was taking prescription drugs and had potentially deteriorated into mental illness.

At Friday's press conference, detectives also gave further detail about some of the explosives that were found inside Paddock's car which was parked in the hotel parking lot.

It contained 50lbs of tannerite - an exploding target which triggers mass explosions when shot at. It is made out of ammonium nitrate and aluminum powder.

McMahill said he did not know what Paddock planned to do with the material but said it had not been modified in anyway to turn into an IED as was reported.

He also dismissed claims that someone else accessed the room using Paddock's key card while his Hyundai was not in the parking lot.

Detectives are working through 'voluminous' amounts of surveillance footage from inside the hotel, he said, but have so far not been able to find any other suspect.

It was also revealed that hotel security guard Jesus Campos, who alerted police to Paddock's specific location after being shot in the leg when he disturbed his killing spree, was on the floor investigating an open door on one of the rooms.

McMahill did not say whether it was Paddock's room which triggered the alarm but he offered a clearer picture of how Campos found himself in the firing line.

'Jesus Campos is a true hero. He was dispatched to what they call a door alarm on the 32nd floor,' he said, clarifying that such an alarm was issued when a door was left ajar.

'He went up there to investigate and as he was doing his job diligently, he came under fire from our suspect.

'He was struck in the leg, and he turned around retreated. He notified his dispatch which was absolutely critical to us knowing the location as well as advising the responding officers as they arrived on that 32nd floor.

'This was a remarkable effort by a brave and remarkable man. I don't think we've done a good enough job of recognizing him and his actions. For that I apologize.

'I want to clear the record. He is an absolute hero,' McMahill added.

The undersheriff also dismissed, again, the repeated claims from ISIS that Paddock carried out the attack in the name of Islamic terror.

The FBI also announced on Friday the launch of a designated campaign to the public for information.

Police are not yet able to tell the survivors who abandoned their belongings at the concert venue as they fled the scene when they will be able to retrieve them.

Members of the FBI walk among piles of personal items at the scene of the mass shooting on Friday

'We have thousands of pieces of property left on the field that day. We are diligently looking at ways to catalogue it, collect it and find the mechanism to return it but we're still days away from allowing that to happen,' he said.

The Route 91 Harvest festival was not the first music event Paddock targeted.

In August, he rented hotel rooms overlooking the Lollapalooza festival in Chicago which Malia Obama attended with friends.

Paddock never showed up for the event despite reserving a suite in the hotel.

A picture is slowly emerging of how he deteriorated into a gambling addiction in recent years, at times blowing $100,000 a time on video poker.

Paddock, whose family said made millions through real estate deals, was considered a low roller but spent enough in the casinos that he frequented to be treated favourably by staff.

His $500-a-night suite at the Mandalay Bay was comped because he always spent so much there.

Such was his relationship with other casinos that his girlfriend Marilou would be given all-expenses paid shopping trips by some of the establishments.

Paddock and Marilou lived in a $400,000 home in a retirement community in Mesquite, Nevada.