A train crashed into a parade float carrying US military veterans and their spouses in Texas, killing four people and injuring 17, police and parade organisers said.
Ten were in critical condition and seven others were listed as stable, Midland police chief Price Robinson said.
The Union Pacific train struck an open trailer as it was being pulled across the train tracks at about 4.30pm on Thursday.
"It's hard to look at. It's a very tragic event, very unfortunate," Robinson said.
The float was carrying veterans through Midland to a banquet in their honour, said Sonny Cleere, an organiser of the Hunt for Heroes event in San Angelo, Texas. Some of those aboard the trailer managed to jump off before the collision, police said.
The parade kicked off a weekend of events, including the banquet and a hunting expedition, to honour wounded veterans, Cleere said. Those events have now been cancelled.
A Union Pacific spokesman said the National Transportation Safety Board was involved in the investigation, and referred all questions to that agency. A board spokeswoman said investigators were on the way to the scene.
Many of the 25 West Texas veterans being honoured served multiple deployments in Iraq and Afghanistan, according to biographies posted on a website created by event organisers.
They were described as having been shot on the battlefield or wounded by improvised explosive devices.
A statement issued by the Pentagon said Defence Secretary Leon Panetta, who is travelling in Asia, "was deeply saddened by news of the tragic accident" involving the veterans and their spouses.