Image caption Scarlett Johansson, Vanessa Hudgens and Jessica Alba have all allegedly been victims of computer hacking

Scarlett Johansson appears to have become the latest star to have private nude pictures leaked on the internet. But how did the images end up online?

The two photos - one which appears to show Johansson topless and another showing her bottom - have spread like wildfire since Wednesday with a huge number of reposts of the images and links placed on sites including Twitter.

News that the pictures had been posted was reported by the Hollywood celebrity website TMZ.com at 06:41 Los Angeles time (14:41 GMT) on Wednesday.

But it's unclear where the images were first published.

According to a blog on the Washington Post newspaper's website, the pictures initially appeared on video site WorldStarHipHop.com. By Thursday afternoon, they had been viewed almost 675,000 times there.

But the Los Angeles Times' Ministry of Gossip blog says they came to wider attention when they appeared on US aggregation website BuzzFeed.

Image caption Hacked photos of Justin Timberlake and Mila Kunis have reportedly appeared online

Even if Johansson was able to find out the exact order of events, it would be of little comfort to the 26-year-old actress.

According to TMZ.com, she does not believe that all publicity is good publicity and has instructed her lawyer to approach individual websites to demand the pictures are taken down.

He will have his work cut out. It now takes an experienced web user a matter of seconds to find the pictures if they so desire.

And it seems millions do.

Gossip website Holy Moly was one of the first UK-based sites to post the pictures, at 13:04 GMT on Wednesday, under the headline: "Scarlett Johansson naked: Either wonderful Photoshop or a very similar lookalike."

It's a celebrity gossip website, if celebrities don't want naked pictures of themselves plastered all over the internet then either don't take them at all or don't have them on your mobile phone Jamie East, Holy Moly

As a result, the photos attracted more than a million page views in one day.

Holy Moly founder Jamie East says his staff first saw the pictures on US-based site reddit.com, which he believes was the original source.

The photos had been published on the user-generated website by 12:00 GMT on Wednesday. One image of Johansson posted on the site, which had attracted more than 2,900 comments from users, has since been removed.

"We had by far the biggest day in Holy Moly's history yesterday - 20 times our normal traffic and the site's currently down because of it," said Mr East, who is also a co-host on Channel 5 Big Brother spin-off show Bit on the Side

Asked what he would say to those who think the publication of stolen photos is an invasion of privacy, he replied: "Nothing at all".

"I'm not here to argue the morals of it.

"It's a celebrity gossip website, if celebrities don't want naked pictures of themselves plastered all over the internet then either don't take them at all or don't have them on your mobile phone."

FBI investigation

Graham Cluley, senior technology consultant at security firm Sophos, agrees that the "golden rule" for stars should be to abstain from taking naked photos.

My guess is that, even if these photos were taken on Scarlett Johansson's phone, she would then maybe have emailed them to somebody Security expert Graham Cluley

"And certainly don't put them anywhere on the internet because you may have a weak password which allows hackers to get in."

He says that if photos taken on a mobile phone remain on a mobile phone, it's much harder for hackers to get hold of them.

"You would normally need physical access to the phone or to install some malware onto it," he adds.

He points out that High School Musical star Vanessa Hudgens had naked photos leaked onto the internet after her web mail account was hacked.

"My guess is that, even if these photos were taken on Scarlett Johansson's phone, she would then maybe have emailed them to somebody.

"Then, either their email got hacked or they were in her sent folder in her online email account - that would be the most natural way."

There's been a frenzy of activity from the male population online looking for these pictures Security expert Graham Cluley

The cases of Johansson and Hudgens, as well as that of actress Jessica Alba and dozens of other stars who have been hacked, are all likely to be part of an ongoing FBI investigation.

Mr Cluley suggests the hackers could be earning money by selling the photos to websites either directly or through intermediaries.

"I would always suggest following the money trail if you want to find out who's responsible," he says.

And he also offers a note of caution to red-blooded internet users.

"There's been a frenzy of activity from the male population online looking for these pictures.

"I would say to people, be very, very careful because if I were a virus writer it would be very easy to send out links saying here are more naked pictures of Scarlett Johansson.

"And then, if you clicked on that link, your computer could then become infected."