LAS VEGAS — As investigators keep filling in the details of Stephen Paddock’s rampage Sunday, doctors, nurses and paramedics are recounting injuries rarely seen in this country.

With Paddock perched on the 32-floor of the Mandalay Bay Resort and Casino and firing military-style rifles onto the crowd below, the scale and degree of physical damage were extreme.

Many of the most critically wounded arrived at the University Medical Center of Southern Nevada, the state’s only Level One trauma center. Over about four hours, it received 104 patients. More than 80 percent were gunshot victims.

Douglas Fraser, the hospital’s chief of trauma surgery, struggled with other doctors there to deal with bullet wounds in torsos and limbs that had shredded human flesh into “unusual patterns,” caused “extreme fractures” and bounced through bodies with horrific force.

“The fractured shrapnel created a different pattern and really injured bone and soft tissue very readily,” he said. “This was not a normal pattern of injuries.”

The devastation that semiautomatic rifles cause is extreme because they put vastly more energy behind a bullet than handguns do. The velocity of a bullet fired from a typical 9mm handgun is 1,200 feet per second. From an AR-15 semiautomatic, the bullet travels roughly three times faster.

A 9mm bullet striking someone in the liver, for example, it might cause a wound an inch wide, said Ernest Moore, a trauma surgeon at Denver Health and editor of the Journal of Trauma and Acute Care Surgery. “But if you’re struck in the liver with an AR-15, it would be like dropping a watermelon onto the cement. It just is disintegrated.”

Survival generally depends on several factors: the position of the body when struck and distance from the weapon; velocity of the bullet and the type used; and location of the entry wound and path the bullet follows before it exits – if it exits at all.

About half of the victims transported to University Medical Center suffered graze wounds, probably from bullets that ricocheted off the ground, Fraser said. Others may have been struck by bullets that passed through other victims. Some were trampled in the panic.

But 30 were in critical condition with direct hits, he said.