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They call themselves the Impact Team and seem to have formed solely to carry out the attack on the infidelity website. There is no evidence of the group stealing data elsewhere before it announced itself with the Ashley Madison attack on 15 July.
Comments made by Noel Biderman, chief executive of Avid Life Media, which owns Ashley Madison, soon after the hack became public suggested it knew the identity of at least one of the people involved.
"It was definitely a person here that was not an employee but certainly had touched our technical services," he told security blogger Brian Krebs.
Since then, little new information has been made public about the hack, leading some to assume that the information Avid had about a suspect would soon lead to an arrest.
But it did not, and now gigabytes of information have been released and no-one is any the wiser about who the hackers are, where they are located and why they attacked the site.
The group is technically pretty competent, according to independent security researcher The Grugq, who asked to remain anonymous.
"Ashley Madison seems to have been better protected than some of the other places that have been hit recently, so maybe the crew had a stronger skill set than normal," he told the BBC.
They have also shown that they are adept when it comes to sharing what they stole, said forensic security specialist Erik Cabetas in a detailed analysis of the data.
The data was leaked first via the Tor network because it is good at obscuring the location and identity of anyone using it. However, Mr Cabetas said the group had taken extra steps to ensure their dark web identities were not matched with their real-life identities.
The Impact Team dumped the data via a server that only gave out basic web and text data - leaving little forensic information to go on. In addition, the data files seem to have been pruned of extraneous information that could give a clue about who took them and how the hack was carried out.
The only potential lead that any investigator has is in the unique encryption key used to digitally sign the dumped files. Mr Cabetas said this was being employed to confirm the files were authentic and not fakes. But he said it could also be used to identify someone if they were ever caught.
But he warned that using Tor was not foolproof. High-profile hackers, including Ross Ulbricht, of Silk Road, have been caught because they inadvertently left identifiable information on Tor sites.
The Grugq has also warned about the dangers of neglecting operational security (known as opsec) and how extreme vigilance was needed to ensure no incriminating traces were left behind.
"Most opsec mistakes that hackers make are made early in their career," he said. "If they keep at it without changing their identifiers and handles (something that is harder for cybercriminals who need to maintain their reputation), then finding their mistakes is usually a matter of finding their earliest errors."
"I suspect they have a good chance of getting away because they haven't linked to any other identifiers. They've used Tor, and they've kept themselves pretty clean," he said. "There doesn't seem to be anything in their dumps or in their missives that would expose them."
The Grugq said it would need forensic data recovered from Ashley Madison around the time of the attack to track them down. But he said that if the attackers were skilled they might not have left much behind.
"If they go dark and never do anything again (related to the identities used for AM) then they will likely never be caught," he said.
Mr Cabetas agreed and said they would probably be unearthed only if they spilled information to someone outside the group.
"Nobody keeps something like this a secret. If the attackers tell anybody, they're likely going to get caught," he wrote.
A lot of data has been released about Ashley Madison but some facts of the breach of the dating website's database remain stubbornly elusive, not least who are the hackers behind the attack?

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It would be very annoying to most people but competition is a daily normality for boxing twins Gavin and Jamie McDonnell, who could become the first British identical twins to win world titles.
He would still be fighting at Donny Dome if I didn't help him out.  He got a leg-up from me
"This is massive because I don't know any other twins in the country that have done what we've done," said Gavin, who is unbeaten in 18 fights and faces Mexico's Rey Vargas, 26, for the WBC super-bantamweight title in Hull on Saturday.
"To be the first to do anything is an achievement in itself. Our kid Jamie has done a few things like that. He was the first European and world champion from Doncaster so I can't get them but to be the first twins means I get a bit of room on the stage."
The charismatic Doncaster duo, both 30, have each collected a pair of British and European titles but at different times.
Of course the main achievement that separates the two is Jamie's current status as the WBA bantamweight world champion - he has also previously held the IBF version.
Gavin turned pro in 2010 - five years after his brother - but has quickly risen through the ranks and sees his next fight for the prestigious green and gold WBC belt as a chance to step out of Jamie's shadow.
"I'm being compared to him all of the time. He's Mr Bee's Knees, who's done everything - it takes the shine off my fights," said Gavin.
"Nobody wants to be a number two at anything. He's been number one all his life and now I'm giving him a taste of his own medicine. He's number two now if I win the WBC title. He's got the other belts - IBF and WBA, but if I can say, 'who's got the best world title?' No comeback."
But with twins there's always a comeback and Jamie duly interrupts: "You then have to defend it five times to be on a level with me.
"You only have that fight because you were in my ear saying, 'Get me in with Eddie Hearn [the McDonnell brothers' promoter],' so I got him a meeting with Eddie and I said, 'help him out Eddie, will you?' He would still be fighting at Donny Dome if I didn't help him out. He got a leg-up from me."
Gavin's task on Saturday is no easy one as he goes up against a man unbeaten in 28 fights, with 22 knockouts. But promoter Hearn reckons the "down and dirty" Hull Ice Arena will give him an advantage.
"We had decent money on the table to go to Mexico for this fight but I thought Hull Arena can do strange things to a man," said Hearn.
"When you go in those changing rooms and walk out, it's a bear pit. If you're going to beat Vargas - a brilliant fighter - you have to make him feel uncomfortable.
"He has to open the door to the changing room and go 'what is this?' We need every advantage we can get in this fight."
He may seem the confident twin now but Gavin does admit he would not be boxing if it were not for his brother's achievements.
"If it wasn't for Jamie I'd be in the pub with the lads and working a 9-5 job," said Gavin, who worked as a plasterer alongside his twin.
"I never had an ambition to become world champion. I knocked boxing on the head at about 16 because I wanted to go out with my mates. But I saw Jamie at all his fights and saw him win titles. I didn't think I would do that but I wanted to have a go.
"Now I'm fighting for a world title and I believe I can win one. My brother's done it and I believe I can do it."
If Gavin cites his brother as inspiration, Jamie can now see the motivational benefit of such a close-knit rivalry.
"We push each other. Now he motivates me," said Jamie, who suffered the last of his two defeats in 2008.
"I've got to step it up otherwise people will say your brother's overtaking you and I can't have that. We're not jealous of each other. We are just born winners."
The pair are typical twins, always trying to get one over on each other in anything, from their follower numbers on social media to who could build the better house. But as their boxing profiles grow rapidly, there remains a problem.
"We get mistaken for each other all the time," said Jamie. "If someone calls me Gavin, I just go with it."
Gavin adds: "There'll be a tweet saying, 'Great to meet world champion Jamie McDonnell' and it was me. That makes me steam a little bit because I'm not a world champion yet."
There have already been world champion twins - the American Charlo brothers, Jermell and Jermall, own IBF and WBC belts in the light-middleweight division. The McDonnells aim to go one step better by one day completing the WBA, WBC, IBF and WBO four-piece set.
"We could get all four between us," said Gavin.
"The question is, whose mantelpiece will they go on? I'm sure we can collect them all in a few years because how many twins have done that? I think every fight for me now will be big fights at the top alongside him."
Can you imagine seeing somebody who is a mirror image of you trying to do your job better than you can every single day?

Caroline Weir gave the hosts a third-minute lead with a long-range shot that was fumbled by England goalkeeper Carly Telford.
Notts responded, edging possession and creating some steady pressure.
But Liverpool made it 2-0 when Natasha Harding curled in a fine shot, and they held on for victory after the break.
The draw for the semi-finals will take place live on BBC Radio 5 Live at 14:45 BST on Monday, 27 March.

Liverpool Ladies scored two first-half goals as they beat Notts County Ladies to secure a place in the Women's FA Cup semi-finals.

