In an emotional piece that Riley Blanks Reed penned for ESPN on Tuesday morning ... she announced Lance killed himself last Wednesday.

"People will want more information," Blanks Reed said of her father's death. "But the fact is we will never truly know why. And we don't need to. All we need to do is remember him, honor him, celebrate him, and pour our love into the family that made him happy."

Lance was 56 years old.

In her letter, Blanks Reed reminisced on all the good times she had with her father ... remembering him as "my person, my idol, my teacher, my best friend and my confidant."

"He encompassed everything to me," she said.

Lance starred at the Univ. of Texas from 1988 through 1989 -- and had such a great two seasons there, he was inducted into the school's Hall of Honor in 2007.

Following his Longhorns days, Lance went on to have careers as a player and an exec in the NBA -- ultimately working for the Suns as their GM from 2010 to 2013 -- before he signed on to be an analyst for the Longhorn Network.

Blanks Reed said while all of his career accomplishments were incredible ... she knew him best for being "a leader and a guardian angel."

"In the darkness of my worst nights," she wrote, "he always picked up the phone."

Blanks Reed added, "Daddy, Lance, LB, my forever hero, has left a void that cannot be filled. And yet, I find solace in the promise to honor him with everything he left us. In the loving embrace of my sister, Bryn, I will remember that we are both forever connected to the remarkable man who was our world."