Brister said there was no indication Reeves was going to take his own life.
Superintendent Tommy Wallis, said the district is cooperating with the investigation, but declined to suggest any reason why Reeves resigned and committed suicide.
"I don't have any idea what Mr. Reeves was thinking," he told the station. "I can't get into anybody's head."
Reeves' brother-in-law, Trey Ainsworth, told the New York Post that Reeves was well-liked throughout the school and seemed to have much to live for.
"There's nothing that indicated that he would do this," Mr Ainsworth said. "We don't know of anything."
Mr Ainsworth added that the family was unaware if Reeves was in any legal trouble or had been suffering from any medical conditions, like depression.
"It's kind of a shock to everyone," he said. "Everybody loved Dennis. He was a great guy. It seemed like in recent years he was like citizen of the year in Kirbyville. The kids loved him, and his teachers loved him, too."
Reeves, according to his obituary, had been the principal at Kirbyville High School for three years. He was also an avid deer hunter and enjoyed camping with his wife, Tammy Kelley Reeves, and their two sons, Austin and Dylan.
"Dennis loved watching his boys play football at KHS and watching the Dallas Cowboys," the obituary read. "He loved restoring old tools and rescuing and refinishing driftwood."
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• Samaritans 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.