Kalen Schlatter and Tess Richey walked hand-in-hand into an alley between two houses on Church Street at 4:14 a.m.

At 5 a.m., Schlatter walked out alone.

Between those two moments on Nov. 25, 2017, captured in less than 20 seconds of black-and-white security video and shown to a Toronto jury, the Crown alleges Schlatter sexually assaulted and strangled 22-year-old Richey after she refused to have sex with him.

The defence at Schlatter’s ongoing first-degree murder trial has suggested someone else could be responsible for killing Richey sometime after Schlatter walked away. Richey was never seen alive again and an officer has testified the motion-activated security videos in the area do not show anyone else entering or leaving the alley area.

Richey’s body was found four days later by her mother and friend at the bottom of a stairwell leading to a basement door near the back of the alley.

Schlatter, 23, has pleaded not guilty.

On Thursday, forensic biologist Melissa Kell testified that Schlatter’s DNA was found in saliva on Richey’s bra and semen on her upper left pant leg.

Earlier on Thursday, one of two undercover officers who were placed in holding cells at a police station with Schlatter after his arrest was recalled to answer further questions from the defence. The officer had previously testified about his part in an undercover operation to get Schlatter to discuss Richey’s death.

Defence lawyer Lydia Riva played a portion of a video of Schlatter being taken through a police office and being put into the cells. In the video you can hear the cell door bang shut and then, she said, it is possible to faintly hear the start of the undercover officer’s conversation with Schlatter through the open office door.

The officer has testified that it was Schlatter who prompted all the topics of conversation, including an initial discussion about the Superbowl. Schlatter had been arrested after watching the game with his family at a theatre. The conversation between Schlatter and the two officers was not audio-recorded.

The content of the conversation has come from the testimony of the two officers, with the defence challenging the reliability and accuracy of their memories of the conversation which took place over about four hours.

Riva said the video of Schletter being led into the cell includes audio of the undercover officer speaking first, saying: “What time is it? Did you just get in here? What’s going on?”

She also suggested the officer said: “Did you watch the football game? Who won, man?”

The officer said he was unable to hear this on the poor-quality audio, and said it also wouldn’t be possible to know if that was the start of a conversation. He said he did hear someone who could have been him say: “I didn’t finish the f---ing game.”

The officer denied that he was pretending he couldn’t hear what was being said in the audio in order to avoid contradicting his testimony that it was Schlatter who began the Superbowl conversation.

Riva also questioned the officer about why he moved a monitoring device he had with him for safety purposes to be as close as possible to Schlatter’s cell. The device allowed the officers’ handlers to listen to the conversation purely for safety purposes and not for the content, the trial has heard.

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The officer said he’d been worried about the device dropping out of his pocket and that it could be muffled in there, but said there was “no rhyme or reason” for why he placed the device where he did, and why he repeatedly moved it and put it in and out of his pocket.

Riva suggested the officer wanted Schlatter’s words to be heard clearly by other officers via the transmitting device, rather than using the device to monitor for any signs of officer distress. The officer denied this.

The jury also briefly heard from Betty Chow, a Centre of Forensic Sciences toxicologist who tested a sample of Richey’s blood for alcohol. She found a blood alcohol level of 206 mg of alcohol per 100 ml of blood, high enough to possibly cause significant cognitive impairment and physical symptoms like slurring and inability to walk properly, she said. The blood alcohol level could have been elevated by 20mg to 30mg due to the decomposition of Richey’s body prior to the sample being taken, she said.

Correction - Feb. 28, 2020: This article was edited from a previous version that misstated which pant leg the semen was found on.