The tiny detail in Adnan Syed's phone records that won the convicted murderer a new trial.

HBO’s four part documentary, The Case Against Adnan Syed, has been packed full of new discoveries and groundbreaking revelations about the case millions have been obsessed with since the award-winning podcast Serial dropped in 2014.

From the podcast we know that in 1999 Baltimore high school student, Hae Min Lee, was murdered – and her ex-boyfriend, Adnan Syed, was convicted of the crime, despite serious doubts from his many advocates.

And in the doco’s first two episodes, we learned startling new information about Don Clinedinst, the man Hae was dating at the time of her murder, and got a much more intense look at Jay Wilds, the prosecution’s star witness.

Watch the full trailer for The Case Against Adnan Syed here. Post continues below…

Now, episode three has revealed the details of a key piece of evidence that led to Syed being granted a second trial – a decision that was subsequently overturned in March this year.

Enter: Susan Simpson.

Simpson is an attorney and advocate for Syed. She became involved in the case when she was approached by Rabia Chaudry, who’d read the blog Simpson had written, analysing Jay Wilds‘ conversation with police.

Chaudry said: “I just emailed her and said, ‘You’re analysing this stuff in a way that nobody ever has. I have all the files, and I’d love to give it all to you.’”

And it was Simpson who, when reading through the files, discovered a vital piece of evidence.

When looking at the phone records and cell tower data, Simpson noticed a section titled “How to read ‘subscriber activity’ reports”. It stated that: Outgoing calls only are reliable for location status. Any incoming calls will NOT be considered reliable information for location.

Considering that it was an incoming call that had been used by the prosecution to link Syed to Leakin Park, where Hae’s body was found, Simpson knew this was huge.

The documentary also reveals that cellular network technology expert, Abe Waranowitz, had not been aware of the disclaimer.

“If I had been made aware of this disclaimer, it would have affected my testimony,” he wrote in a letter to Simpson.

And so it was on that evidence – not on the possible alibi provided by Asia McLain – that a fresh trial was granted.

Disappointingly for Syed, that ruling was overturned in the Maryland Court of Appeals last month, and his murder charge was reinstated.

On Friday, Syed’s attorney, Justin Brown said he now plans to ask the U.S. Supreme Court to review the case.

You can watch all four episodes of The Case Against Adnan Syed on SBS on Demand now.