The parade had been scheduled to end at a "Hunt for Heroes" banquet honoring the veterans. The wounded service members were then going to be treated to a deer-hunting trip this weekend. The events were canceled.
The events were organized by Show Of Support, a local veterans group. Its president, Terry Johnson, did not immediately return an email for comment and his phone number was unlisted; the phone rang unanswered at the group's offices.
Stout, also the police department's spokesman, said he had no information about the individuals who died or the driver of the truck.
Lange said Union Pacific is offering help to the community and victims' families, as well as peer-to-peer counseling for the train crew, who did not sustain any injuries.
"There is going to be a very thorough investigation," Lange said. "It's obviously a very tragic incident."
The National Transportation Safety Board also is investigating, NTSB spokesman Peter Knudson said.
Secretary of Defense Leon Panetta "was deeply saddened by news of the tragic accident involving veterans heroes and their spouses in Midland," Pentagon spokesman George Little said in a statement. "His thoughts and prayers are with the families of the victims, with those injured in this incident, and with the entire community."
Midland is about 320 miles west of Dallas.
Terry Wallace reported from Dallas. Associated Press writer James Beltran also contributed to this report from Dallas.