Carrie Underwood Reacts to 'Unfathomable' Death of Musician Brett James

"Brett was the epitome of 'cool,'" Underwood wrote of James, with whom she frequently collaborated, after he died this week in a small plane crash.

Carrie Underwood is opening up about her friendship with Brett James, who died in a plane crash on Thursday.

The "Remind Me" singer -- who frequently collaborated with the musician -- took to Instagram to share memories of their time together.

"Some things are just unfathomable," she began her September 19 caption on Instagram.

"The loss of Brett James to his family, friends and our music community is too great to put into words," Underwood continued. "Brett was the epitome of 'cool.' I see him in my mind riding up to my cabins to write on his motorcycle [and] his hair somehow perfectly coiffed despite being under a helmet for however long."

She added: "He was a good guy. I remember writing with him on a song that he basically wrote 75% of and had ready when he walked in the room. We filled in the blanks and added a little melody and I told him after that I didn't feel right splitting the credit evenly when he did most of the work. He wouldn't have it. He insisted that everything be equal. He was just that kind of guy…"

She noted that his death left "a hole in all of us that [she feared] won't ever go away."

"It will forever be a reminder that this life is but a moment," added Underwood.

The 42-year-old ended the caption with "love you, man. I'll see you again someday."

He cowrote Underwood's "Jesus, Take the Wheel," "Flat on the Floor" and "Cowboy Casanova."

James -- who also worked with the likes of Taylor Swift -- died in a small plane crash on Thursday in Franklin, North Carolina at around 3 p.m.

Per NBC, the North Carolina State Highway Patrol confirmed James and two other people aboard the plane -- including Melody Carole Wilson, his wife, and Meryl Maxwell Wilson, James' stepdaughter -- were all killed.

James was 57 and also had four children from a previous marriage.