A scene in Making a Murderer has led viewers to uncover a piece of crucial evidence.

If you’ve watched the series you’ll know that the most crucial evidence is the discovery of the key to Teresa Halbach’s car in Steven Avery’s house.

The singular key on a blue lanyard contains his DNA and no other traces.

However, viewers have pointed out that in this photo of Teresa Halbach, you'll see that in Halbach's closed hand there is not just one key. In her hand, there are multiple keys.

Obviously, the key could be a spare that Halbach kept but there is a significant level of suspicion surrounding the event.

Speaking to Rolling Stone, lawyer Jerry Buting said:

"Dean and I worked on that case for 14 months before that trial, the last six months of which I did nothing but work on this case. We had 25,000 pages of discovery and police reports to go through, we had hundreds and hundreds of hours of tape-recorded phone calls and interrogations and aerial footage, every salvage yard and expert opinions. But we were only two minds. What I’m discovering is that a million minds are better than two. Some of these people online have found things with a screen shot of a picture that we missed.

There’s a picture of [murder victim Teresa Halbach] that I’ve looked at a thousand times that shows her standing in front of her vehicle, the RAV4. She’s got a camera in one hand and she’s also got keys. And it’s like a key ring. You can see there’s a bunch of keys there like a house key, probably a photography studio key, maybe her parents’ house key. And those were never recovered. Instead, we found this single key.

Now we did challenge that, about how unusual it was, why is she walking around with one key? But I don’t think I caught the fact that there was a photograph showing what she really carried around with her is a bunch of keys. And none of those keys were ever found."