Carl-Fredrik Arndt and Peter Jonsson, the two Swedish Ph.D. students who found Brock Turner sexually assaulting an unconscious woman behind a dumpster at Stanford, said they immediately knew something was wrong.

Carl-Fredrik Arndt and Peter Jonsson, the two Swedish Ph.D. students who found Brock Turner sexually assaulting an unconscious woman behind a dumpster at Stanford, said they immediately knew something was wrong.

Nearly a year and a half ago Arndt and Jonsson were biking to a party on Stanford's campus when they saw Turner, 20, on top of his victim behind a dumpster. They managed to tackle Turner as he ran away and they called the police.

The two men were key witnesses in the case against Turner, who was eventually found guilty of three counts of sexual assault and was sentenced by Judge Aaron Persky to a six-month sentence in county jail and probation. He said a longer sentence in prison would have a "severe impact" on the Stanford athlete.

Since the victim's letter was published on Friday — followed by other letters from Turner's father and friend defending him — people all over the world, including public figures and celebrities, have come out in support of the victim while expressing outrage over Turner's short sentence.

In the letter, she described the two students — whom she has still never met — as "heroes."

Arndt described the night of the assault to the Swedish news outlet Expressen on Tuesday, saying that they read the letter and were very moved. (Arndt did not immediately return BuzzFeed News' request for additional comment and Jonsson declined, saying he is not currently speaking to press.)

As Arndt and Jonsson approached the dumpsters where Turner was attacking his victim, they said they immediately knew something was wrong.

"We saw that she was not moving, while he was moving a lot," Arndt said in Swedish. "So we stopped and thought, 'This is very strange.'"

The two graduate students quickly decided to approach Turner to see what was going on. Jonsson approached him first, Arndt said, while he followed.

"When he got up we saw that she still wasn't moving at all, so we walked up and asked something like, 'What are you doing?'"

Arndt told CBS News he saw Turner "aggressively thrusting his hips into her."