TORONTO -- A Toronto officer on trial for killing a teen on an empty streetcar believed the youth was ready to "fight till the end" when confronted by police on a summer night two years ago.

Const. James Forcillo testified Thursday that his concerns about Sammy Yatim appeared founded when he saw the knife-carrying teen jerk his switchblade towards him.

"In my mind things just crystallize -- he's coming off that streetcar to attack me," Forcillo said, noting that at no point did he believe the teen was surrendering. "Mr. Yatim was in it to fight till the end."

Forcillo has pleaded not guilty to second-degree murder and attempted murder in the July 2013 death of 18-year-old Yatim.

The officer's testimony marks the first time the public has heard an explanation for his actions, which triggered outrage across the city.

At the crux of the trial is whether Forcillo was justified in shooting Yatim.

Crown prosecutors argue Forcillo's actions during the incident weren't necessary or reasonable. Forcillo's lawyer contends his client's actions were justified and carried out in self-defence.

The jury has heard that Yatim had taken the drug ecstasy at some point before boarding a streetcar where he eventually pulled out a small knife, causing panicked passengers to rush off.

On Thursday, under questioning from his defence lawyer, Forcillo took the jury back to the night where he came face to face with Yatim.

When he saw Yatim holding a knife, Forcillo said he pulled out his gun -- as he was trained to -- because knives are considered a "deadly threat."

Forcillo also explained that he believed his gun was his best option because hand-to-hand combat would have been too dangerous, using his baton would have brought him too close to Yatim and pepper spray, in those circumstances, would have been ineffective.

Forcillo yelled "drop the knife" repeatedly, but Yatim didn't comply, court heard.

"This is a person who doesn't want to talk, this is a person who wants to fight," Forcillo said of his interpretation of the situation. "His jaw was tense, his face was flushed, his eyes wide, his body tense."

Forcillo also said he noticed Yatim seemed "not at all fearful." in the face of a gun.

While he considered the possibility that Yatim was impaired by drugs, Forcillo said the teen was nonetheless "in the moment."

"Mr. Yatim knows what's going on and he's making decisions," he said. "They're bad decisions but he's making them."

Forcillo didn't consider closing the streetcar doors to contain Yatim because he wasn't sure if there was anyone else on the vehicle, court heard, nor did he want to give Yatim the option of hijacking the vehicle.

The standoff between Forcillo and Yatim -- which lasted some 50 seconds -- escalated after Yatim took a few steps back from where he had been standing at the top of the vehicle's front steps.

At that point, Forcillo warned Yatim that if the teen took a step forward he would be shot.

"He goes still and he takes a deep breath, and to me it looks like he's making a decision," Forcillo recalled. "What I see next is he flicks the knife at me."

Forcillo said he believed Yatim was "fully capable" of attacking him and believed the teen could reach him in about two seconds.

"Did you want to shoot Mr. Yatim as he came forward?" Forcillo's lawyer asked.

"Of course not," Forcillo said. "I yelled 'drop it.' A last-ditch attempt to maybe not make this decision," Forcillo said.

"What was the last thing Mr. Yatim said in response?" his lawyer asked.

"No, he said no," Forcillo recalled.

The jury has seen videos and heard audio that show Forcillo then fired three bullets at Yatim, causing the teen to crumple to the floor. After a brief pause, Forcillo fires six more bullets.

Forcillo said he fired the second volley because he believed Yatim was still trying to attack him.

"He scooped the knife back into his hand," Forcillo said. "None of the videos show what I see."

A use-of-force expert called by Crown prosecutors has testified that Forcillo had a number of other alternatives to lethal force before he shot Yatim but didn't use them.

Forcillo dismissed those suggestions -- which included lobbing a baton or a paint can at Yatim, or waiting a few moments longer before firing -- saying that they would have either enraged Yatim or allowed the teen to attack.

After the second volley, Forcillo said he planned to board the streetcar to get the knife out of Yatim's hands when the teen got a "wide-eyed look."

"It's scary, so we stopped," he said.

The jury has heard that Yatim was killed by Forcillo's first three bullets, which hit him in the heart, spine and the arm.