An Oregon man who died and “dissolved” after falling into a boiling, acidic hot spring at Yellowstone national park last June had been looking for a place to swim, officials investigating the incident have concluded.

Colin Scott, 23, was hiking through a prohibited section of the park on 7 June with his sister, Sable, when Scott fell into a hot spring “and did not get out”, according to a report released by the National Park Service on Thursday.

Deputy chief ranger Lorant Veress told a local news station, KULR-TV, the pair were searching for a place to “hot pot”, the illegal practice of swimming in one of the park’s thermal features.

“[They] were specifically moving in that area for a place that they could potentially get into and soak,” Veress told the station.

Sable Scott was filming a video of the pair “intentionally” walking off the Norris Geyser Basin’s boardwalk, according to the report, when her brother fell in.

“The smartphone recorded the moment he slipped and fell into the pool and her efforts to rescue him,” the report said. There’s no cellphone service at the basin, according to the report, so Sable Scott went back to a nearby museum for help.

The incident report was released in response to a Freedom of Information Act request by KULR. Officials withheld the video clips from release.

When park officials arrived, portions of Colin Scott’s head, upper torso and hands were visible in the hot spring.

“Due to the report of the individual not previously visible, a lack of movement, suspected extreme temperatures, and indications of several thermal burns, the subject was determined to be deceased,” US park ranger Phil Strehle wrote in a 9 June report. A v-neck-style shirt was visible, he said, and “what appeared to be a cross was visible and resting on the subject’s face”.

Rescuers were unable to safely recover Scott’s body, due to the “volatile” thermal area and an incoming lightning storm. When officials returned the following morning, Scott’s body was no longer visible.

“The consensus among the rescue/recovery team … was that the extreme heat of the hot spring, coupled with its acidic nature, dissolved the remains,” a report said. A wallet and a pair of flip-flops belonging to Scott were recovered.

Water temperatures at the basin typically reach 199F (93C); at the time Scott’s body was recovered, the report said, rescuers recorded a temperature of 212F, at which point water begins to boil. Warning signs are posted around the area to direct visitors to remain on the boardwalk.

Scott’s sister told investigators that he was visiting her from Portland, Oregon, and had recently graduated from college before coming to visit her.

No citations were issued, the report said.

The National Park Service advises visitors on its website to stay on boardwalks and trails in thermal areas.

“Hot springs have injured or killed more people in Yellowstone than any other natural feature,” according to the service.

• This article was amended on 18 November 2016. An earlier version used the surname Strehle in one instance where Scott was meant.