She snapped on a pair of gloves. ‘It’s named after rhesus monkeys, because they were initially used to create the anti-serum for typing blood samples.’

‘Oh,’ Gary said. ‘Poor monkeys.’

Sara laid out some clean paper towels and emptied the kit onto the counter. She set aside the alcohol swab and lancet because they weren’t testing a live subject. She separated the four Eldon sticks—basically plastic Q-tips—and the tiny bottle of water that came with the kit. She told Gary, ‘Write on the card where the first sample came from.’

Gary took a pen from his pocket and wrote LEFT STAIR TWO IMPACT, then the address for the building, date and time. His gold medallion tapped against the desk. Sara assumed he hadn’t met Amanda yet. She had once slapped a ruler to the back of Will’s neck to make sure that his hair was the regulation one inch off his collar.

Sara put on her glasses. She laid the card flat on the paper towels. She squeezed a pin drop of water onto the four separate reagents in each circle. Gary opened one of the test vials, which contained a glob of tissue, probably scalp. Sara used a glass pipette to collect some blood. She dabbed the blood at the bottom of the control well. She used the Eldon stick to mix the blood and reagent inside the margins of the printed circle.

Gary said, ‘Would it be clotting already?’

‘Not the control. It should always look smooth.’ Sara dropped more blood onto the first circle, marked ANTI-A, and swirled it around with a fresh stick. Then she did the same for anti-B and -D. She told Gary, ‘Next, you turn the card on its side, hold for ten seconds, then upside down for ten seconds, and so on until you make a full revolution to mix the blood with the reagent.’

Gary said, ‘It looks like the B is clotting.’

He was right. There were patch-like red clumps inside the B circle.

‘There’s no clotting in the D circle,’ Gary said. ‘That means it’s B-negative, right?’

‘Correct,’ Sara told him. ‘Well done.’

‘Do we know the blood type for Mrs Trent?’

Sara felt the name like a punch to her throat. ‘She goes by Polaski.’

‘Oh, sorry. My bad.’

‘I haven’t received her blood type yet.’ Sara checked her phone to make sure a text hadn’t come in from Amanda. She wondered again if something had happened. Will had a habit of agreeing with Amanda, then doing whatever he wanted. Sara used to find that attractive.

Gary asked, ‘Is Mrs Polaski’s DNA on file from when she was a cop?’

Instead of telling him they could probably find an intact sample on Sara’s lipstick, she answered, ‘It’s unlikely unless she was a rule-out at a crime scene. She worked vice, so there probably wasn’t a need.’ Sara forced her thoughts to stay on the task at hand. ‘DNA is the gold standard, but the typing is a significant finding. B-negative is found in only two percent of Caucasians, one percent of African Americans and well under a half a percent in the remaining ethnic groups.’

‘Wow. Thank you. That there is some mad science, Dr Linton.’ Gary took out his pen and filled in the next card without being asked. His letters were neat capitals that easily fit in the square provided. LEFT STAIR BLOODY FOOTPRINT A.

He said, ‘So, the water first, right?’

‘Just a pin drop.’ She kept silent while Gary processed the next kit. He really was a fast learner. When he mixed the blood, his margins inside the circles were better than hers. He started to turn the card, holding it in place for ten seconds before turning it again, then again. As before, the blood clotted on B-negative.

She told him, ‘Type the sample from Harding’s neck.’

Gary had taken a swab because there wasn’t a lot of blood. He had to use a blade to cut the cotton tip into sections, then use water to free the blood. He went through the same steps with the card. This time, only the circle for D clotted. He asked, ‘Did I do something wrong?’

‘He’s O-positive, the most common blood type for Caucasians, but the important part is this makes Harding a definite rule-out for the footprint and the spatter on the stairs.’ She handed him another kit. ‘Let’s try the sample of blood from the room where Harding died.’

There was a loud knock on the door. Both Sara and Gary jumped at the noise.

‘Good Lord God.’ Charlie held up his camera as he climbed into the van and slid down to the floor. ‘I thought I was going to burst into flames inside that room.’ He closed his eyes and breathed the cold air for a few seconds.

Gary started the next kit. Sara handed Charlie a paper towel to wipe his face. He was soaked through with sweat. They would need to get some fans in the building before they continued. It was August. Even tonight when the sun set, the temperature would only dip a few degrees.

‘Okay.’ Charlie tossed the paper towel into the trashcan. ‘I’ve been activating the luminol inside the other rooms.’

Sara nodded. Luminol was activated by a black light that made the enzymes in blood glow an ethereal blue. The reaction lasted for a few seconds, and only happened once, which was why it was important to have a camera to record the process.

She asked Charlie, ‘Anything good?’

‘Oh yeah. I’ve got it right here.’ Charlie switched on the LED on the back of the camera and started toggling through the pictures. ‘By the way, I found some blood spray on the unicorn, which could mean the bullet went through somebody.’

‘A lot of spray or a little spray?’

‘More like a sneeze.’

‘That’s not enough to test with the EldonCard. We’ll have to go with DNA.’ For Gary’s sake, she added, ‘There’s no time stamp on blood. Could be some raver sneezed out some blood three months ago.’

Charlie said, ‘Nobody knows the trouble that unicorn has seen.’ His thumb worked the scroll on the camera. Rorschachs of bright blue spatters and splatters flashed across the LED.

‘Dr Linton?’ Gary held up the card he’d just processed. ‘More B-negative.’

Charlie asked him, ‘By any chance, did you take a sample from the second room from the left stair?’

‘Yes, sir.’ Gary checked the vials. ‘I found some blood on the floor, back right corner. And I triple-checked the label before I went to the next one, just like you said.’

‘Good boy.’ Charlie said, ‘Type it for me, please.’

Gary waited for Sara to give him the nod to proceed.

She asked Charlie, ‘What’s going on? Did you find something?’

‘Oh, I found something.’

Sara wasn’t one for cliffhangers, but she let Charlie have his fun. For the most part, forensic work was the least glamorous part of policing. It wasn’t like on television, where impeccably dressed, beautiful crime scene techs plucked clues from thin air, waved around guns, interrogated the bad guys, then carted them off to jail. Fifty percent of Charlie’s job was paperwork and the remaining fifty percent had his eye to either a camera or a microscope. He had probably found an unusual pattern of spatter on a ceiling, or the forensic Holy Grail: a viable fingerprint left in fresh blood.

‘There it is.’ Charlie sounded triumphant. He held out the camera so that Sara could see for herself.