Who is the serial killer in ITV's The Loch? Meet the suspects...

• Meet the cast of The Loch
• The Loch review: there are no hidden depths in this ITV murder mystery
• RadioTimes.com newsletter: get the latest TV and entertainment news direct to your inbox

But who did it? Here is the evidence against each suspicious character:

Craig Petrie

Craig Petrie, played by Alastair Mackenzie, is the head of Lochanfoy college.

What is the evidence against Craig Petrie?

– He was the one who discovered the body, while apparently on a jog in his lunch hour. But we know that he took Niall's phone after finding his corpse and later chucked it as far out into the Loch as he could manage. A witness says she saw him put something into his pocket, but Craig lies and told police it was his own phone – which conveniently wasn't working.

– He keeps his home office locked and is suspiciously antagonistic towards his wife when she mentions the murder.

– His handwriting matches the imprint of the writing on a newspaper found where Niall Swift was murdered.

– When the police break into his office, they discover he has been secretly counselling (and videoing) local teens including Jonjo and Dessie. Forensic psychologist Blake Albrighton reckons the questions he is asking are manipulative and controlling.

– When Niall's phone records finally come through, the police find a huge spike in calls between him and Craig in the weeks before the murder. Craig is cagey and confrontational when asked to explain this.

– At the end of episode four he receives a mysterious note under the door: "Need £80 – leave it where you used to meet Niall Swift – 11pm tomorrow." Unless it's a trap, that seems to confirm that it was Craig who sent Niall the text message asking him to meet before his death.

Dessie Toner

Dessie Toner wasn't really in the picture at all until he took a hunting rifle on a school trip, killed his bully Baxter, shot his friend Kieran and then killed himself. He is a kid at the local sixth form college and the son of Annie's best friend.

What is the evidence against Dessie Toner?

– Well, he did kill Baxter in a brutal premeditated murder. Petrie's tapes show he was angry and troubled for quite some time.

– Annie discovered a bloody hammer and a tool that could have been used for extracting brains in the loft space above Dessie's room.

BUT...

– Those may be the murder weapons, but could he have been hiding them for someone else? Could he have been framed?

– His motive for killing Baxter is quite clear, as is his own suicide. But none of the other murders make sense – they are the work of a calculated serial killer, not an angry and bullied school shooter.

Leighton Thomas

Ex-offender Leighton Thomas (William Ash) killed two men 20 years ago and served his time, entering prison almost illiterate and emerging with a degree. He runs a local boat tour business and few local people know about his past.

What is the evidence against Leighton Thomas?

– Considering Leighton's past, he's a suspect by default: he was convicted and jailed for two murders.

– He initially provided an alibi proving he had taken tour groups out at the time of Niall's murder, but Annie later investigates and finds those ticket stubs are a year out of date. When asked why he lied, Leighton says he didn't want everyone to know where he was all the time.

– When his old accomplice Oliver Tench pays an unexpected visit, he keeps this a secret from the police. Tench says he knows it was Leighton who killed Jonjo because he recognises his murder method: a hammer blow to the head. The only thing working in Leighton's favour is that he protects both Annie and Alan from being murdered by Tench, implying that he really has turned over a new leaf.

Dr Simon Marr

Dr Simon Marr (John Heffernan) is the local doctor, a recent widower and a father of one.

What is the evidence against Dr Simon Marr?

– Niall Swift was at Dr Marr's house to give a piano lesson to his daughter shortly before the murder. But Christian Dr Marr fired him in a heated conversation that suggested he was homophobic towards Niall. He handed him a CD that he'd snapped in half and told him not to come back. Why?

– During this conversation Niall told Dr Marr: "You're a hypocrite. I taught Bethan McGrelish." Later Evie discovers secret photos of Dr Marr and Bethan indicating that the two are having a secret affair. It turns out Niall knew about Dr Marr's love affair with the young Bethan. Oh, and to make things even more complicated Bethan is pregnant with Marr's baby, but Bethan's mother has been hiding her daughter and pretending to be pregnant herself so she can raise the kid as her own.

– He gets very shirty with DS Annie Redford when she hears about the argument and starts asking questions, refusing to cooperate because she's not technically working on the case.

– Teenager Evie thinks Dr Marr could be the culprit because he once molested her during a medical exam, though it seems he may be innocent on this one - he was just thoroughly checking her breasts for cancer.

Jordan Whitehead

Jordan is Kieran's supposedly-paralysed brother.

What is the evidence against Jordan Whitehead?

– This is pretty tenuous, but it could explain the weird storyline around Kieran's brother Jordan Whitehead. Supposedly he is paralysed after a cerebral aneurism, but we know he's been waking up recently – much as his mother tries to stop this by secretly injecting drugs. PC Jason Denny visited the Whiteheads' house on his mission to take DNA from every young-ish adult male in town, but when he realised Jordan was bedbound it looks like he left without taking the sample. It's a ludicrous theory, but could Jordan actually be involved in some way?