An American hiker visiting Spain’s Pyrenees mountain range has been missing for eight days, authorities say.

Software developer Cole Henderson was vacationing in Spain when he set out for a hike near Monte Perdido on July 9 in the Ordesa and Monte Perdido National Park. The 27-year-old has not been seen or heard from since, USA Today reported.

Cole HendersonFriends called Henderson an “experienced traveler” and said he shared his full trip itinerary with them before traveling to Spain. He is believed to have been wearing this orange backpack when he began his hike on July 9, Max Senoff/Facebook

Henderson’s friends reportedly became concerned when he missed his July 13 return flight to the Netherlands, where he is living, and reported him missing to Spanish authorities. The following day, authorities launched a search and rescue mission in the Monte Perdido region, deploying helicopters to scan the area.

Henderson’s car was found parked in Torla, a village near the entrance to Mount Perdido, and a receipt for fuel, most likely intended to power a camping stove, was found inside the vehicle, The New York Times reported.

Ordesa y Monte Perdido National ParkHenderson is believed to have entered Ordesa y Monte Perdido National Park on July 9. Friends say Henderson planned to hike through the Ordesa Valley on his way to climb Monte Perdido. Sergi Reboredo/VW Pics/Universal Images Group via Getty Images

Max Senoff, a friend of Henderson’s who also lives in the Netherlands, shared additional details about his disappearance in the Facebook group “Hiking Trails in Spain” on July 15.

According to Senoff, he last heard from Henderson when the hiker texted him the afternoon of July 9 to say he was putting his phone in airplane mode and beginning the hike. Senoff later told the New York Times that Henderson had shared his full trip itinerary with friends before arriving in Spain. He also wrote that Henderson was hiking alone.

Eric Simon, who attended Rhodes College with Henderson, wrote on Facebook that his friend was an “experienced traveler.”

Simon also shared an illustrated map of the mountain range, tracking what friends believe to be Henderson’s planned hiking route based on the itinerary he shared.

Ordesa Valley MapEric Simon, a friend of Cole Henderson, shared a map detailing the hiker’s planned route through the Ordesa Valley. Eric Simon/Facebook

According to Simon, Henderson was traveling east from Torla and likely planned to hike through the Ordesa Valley, camping at various stops before climbing Monte Perdido, returning to the village by bus or taxi. He is believed to have been carrying an orange hiking backpack when he started the trip.

Mount Perdido is the third-highest peak in the Pyrenees, reaching 11,007 feet above sea level. Climbing the mountain is considered somewhat difficult and may require specialized gear, but can typically be done in a single day, CNN reported.

While the mountain range has had mostly sunny weather since Henderson’s disappearance, hikers began reporting concerning storms around July 15. Weather forecasts for the region predict more thunderstorms and strong winds over the next days.

Torla, SpainHenderson’s phone was last pinged in Torla, a village near the entrance of Monte Perdido, where the hiker texted friends on July 9 that he was putting his device in airplane mode and beginning the hike. Raquel Maria Carbonell Pagola/LightRocket via Getty Images

Henderson had been vacationing in Spain for several days when he arrived in Torla and had previously attended the Running of the Bulls in Pamplona.

According to his Facebook page, the hiker grew up in Delaware and worked for some time as a software analyst in Wellesley, Massachusetts, before relocating to Amsterdam.

The Daily Beast has contacted the Spanish police, the Ordesa and Monte Perdido National Park, and two of Henderson’s friends for comment on his last whereabouts and the difficulty of his intended route.