OWINGS MILLS, Md. -- Baltimore Ravens offensive lineman John Urschel, a doctoral candidate in applied mathematics, abruptly announced his retirement from football at the age of 26 on Thursday, just before the first full-team practice of training camp.

His decision, which was announced by the Ravens, comes two days after a medical study indicated that chronic traumatic encephalopathy (CTE) was found in nearly 99 percent of deceased NFL players' brains that were donated to scientific research.

A team source said Urschel's decision was linked to the results of the study.

Coach John Harbaugh said he was surprised when Urschel called him 90 minutes before practice to inform him of his retirement.

"That was out of the blue," Harbaugh said. "He had been working hard. He was working on his snaps all summer. He was doing a great job. It was definitely a lightning bolt that way."

Harbaugh said Urschel explained his decision, but he left it up to Urschel to provide those reasons himself.

"He said he's going to retire from football, [and] that was something that's been on his mind for quite a while and throughout the offseason," Harbaugh said. "That's what he decided to do. We respect John tremendously."

Friday, Urschel sent a text to The Undefeated's Domonique Foxworth, who read part of it on ESPN's First Take.

The text said: "I guess the biggest thing I'd want to say is that I'm excited to focus on my mathematical career full time. And to finally be at MIT full time. A place that I love so much. And when I really thought about it, it didn't make sense to play this season, when the thing I'm most excited about is mathematics right now.

"But that I love the game, love football."

On Thursday, Urschel issued a statement:

Urschel is pursuing his doctorate at the Massachusetts Institute of Technology in the offseason, focusing on spectral graph theory, numerical linear algebra and machine learning. He was expected to compete for the Ravens' starting center job in training camp.

In January, Urschel told HBO's "Real Sports With Bryant Gumbel" that his passion for playing football outweighs the risks of suffering head trauma.

"I recognize that this is somewhat irrational, but I am doing it," Urschel said. "It's more important to me that I'm able to do the two things I love. I don't know if people have really done things that I've done before. I don't know if they'll do it after me. But I enjoy carving out my own path and not listening to what people say I can and I can't do."

In August 2015, Urschel suffered a concussion when he went helmet-to-helmet with another player and was knocked unconscious.

"I think it hurt my ability to think well mathematically," Urschel said. "It took me about three weeks before I was football-ready. It took me a little bit longer before my high-level visualizations ability came back."

Safety Eric Weddle called Urschel a great competitor and teammate.

"As a team and as a leader, you can't worry about the outside or what individuals do," Weddle said. "We're going to go out and play with who we have and move forward. We wish him the best and know he's going to do great things. But it's not with us and we have to move forward."

Urschel was recently named to Forbes' "30 under 30" in the field of science. He has published six peer-reviewed mathematics papers to date and has three more ready for review.

Because of his unique abilities in football and math, Urschel has appeared in a handful of national commercials, including one with Houston Texans defensive end J.J. Watt.