A witness told 7NEWS that people were climbing atop a 300-foot iconic rock at Red Rocks Amphitheatre, possibly triggering a rockfall that sent four concertgoers to the hospital early Sunday.Without a doubt, I can confirm that I saw two (people wearing) T-shirts, climbing on the edge of Creation Rock," Jeremy Hewett said.Creation Rock is a jutting monolith that frames the north side of the amphitheatre. A twin 300-foot monolith, Ship Rock, flanks the venue's south side.Hewett said he saw climbers during the concert despite signs warning "No Climbing" and penalties that include $1,000 fines and potential jail time."I think that someone (climbing on the rock) either fell and started a small rockslide, or it was intentional," Hewett said. "Something needs to be done about people climbing around up there during shows."The cascade of rocks that left stunned concertgoers bloody with severe head injuries and broken limbs happened as the rock band Sound Tribe Sector 9 was doing an encore at about 1 a.m., witnesses said.Rocks rained down on people sitting near the front, left side of the stage, people said."I saw a guy with what seemed to be a large split, directly in the top forward of his head," Hewett.Witness Jennifer Dvrik said, "I saw a guy down on the ground injured. There was blood all over the ground, too."Hewett said he was sitting close enough to the rockfall that a "big rock that hit the kid in the head landed at my feet." He said another young person suffered a compound arm fracture with "bone sticking out of the kid's arm."Denver police spokesman Detective John White said four people were transported to hospitals, including one who remained in critical condition Sunday evening. Three other people had minor injuries and said they would seek treatment on their own, he added.Jennifer Ackerman, 34, who suffered a head injury, had improved to fair condition and was awake and talking on Sunday night, a spokesman at St. Anthony's Central Hospital said."Jennifer is doing much, much better tonight," sister-in-law Kelly Ackerman said in an email to 7NEWS. That's despite having a fractured eye socket, cheekbone and nose and several staples in her skull, Kelly Ackerman added.White said police checked Creation Rock after the rockfall and did not find anyone on the big rock. But he said that someone might have climbed down undetected. Yet, a fire official, who treated the injured, said there's a history of people breaking rules against climbing at Red Rocks."There's access up there into a cave that patrons have snuck into in the past," said Lt. Brendan Finnegan of the West Metro Fire Protection District. "People have been removed from there in the past. So, the potential is there."Finnegan said loose debris was removed from Creation Rock after the incident and the venue is safe.Hours after the incident, firefighters and community members held a 9/11 Memorial Stair Climb at the amphitheatre at 7 a.m. Sunday to honor those who sacrificed their lives during the Sept. 11, 2001, terrorism attacks.Red Rocks is located in Jefferson County, but is owned by the city of Denver.Kristin Rust, spokeswoman for the Denver Office of Cultural Affairs, said she did not know of a prior rockfall incident at Red Rocks.While the venue was deemed safe Sunday, Rust said officials would inspect the rock area on Monday.