A fire cadet collapsed and died during training, officials said.
League City officials said Donald Mize, 62, had participated in a training exercise at the city's training facility in the 700 block of North Kansas Avenue shortly before he collapsed on Wednesday evening.
"They said he wasn't complaining of any problems, that they had been working and he sat down to rest a minute and his heart just stopped," Mize's wife, Theresa, said.
Mize was taken to Clear Lake Regional Medical Center, where he died.
"This is a very tragic loss for the family, the city and the entire community. Our thoughts and prayers are with the Mize family and the League City Volunteer Fire Department," Mayor Tim Paulissen said.
Mize did everything that a firefighter would be expected to do on the job.
"Dragging hoses, dragging dummies, crawling in and over objects, swinging axes, lifting hoses," League City Fire Department Chief Brad Goudie said.
Mize was a graduate of the Citizen's Police Academy and a member of the Texas State Guard.
Mize passed a full physical and showed no problems before he was accepted into the fire academy, officials said.
"That's why this whole thing was such a shock for us -- he is the most healthy person that we know," said Theresa Mize.
Donald Mize met his wife while teaching in the Clear Creek ISD. After his retirement in May, he served as a substitute teacher, shuttled students on field trips and wrote grants. He wrote a grant that got his school the Channel 2 weather bug.
Donald Mize's love of Galveston history earned him the honor of naming several streets on the island.
The city of League City staff and elected officials will wear red and black ribbons in honor of Donald Mize. City flags have been lowered to half-staff.
"I wanted him to do what was going to make him happy," Theresa Mize said.
The cause of Donald Mize's death is under investigation.
Funeral arrangements have not been finalized.