Local hunting guides fanned the lights of their mobile phones across the ground to search for where the ram-like creature fell.
The US president’s son asked them not to dismember the animal on the spot, but instead to carry it away on an aluminium sheet to keep its fur and horns intact.
ProPublica described the August excursion in a report that relies on records and interviews to allege that the president’s son received special treatment from the Mongolian government just weeks after US and Mongolian officials met at the White House.
In Mongolia, permits to shoot and kill an argali, which are prized for their tusks and meat, are determined largely by politics, connections and money, experts said.
Mr Trump Jr received a permit after his hunt – which is reportedly a rare occurrence.
Amgalanbaatar Sukh, a scientist who heads an argali research centre in Mongolia, said that high-level government contacts often decide who gets hunting permits in ways that are opaque to almost everyone else.
The government reportedly authorised 86 permits to be issued in this year’s hunting season, which runs from 1 July to 30 September.
Mr Trump Jr also met privately with the Mongolian president, Khaltmaagiin Battulga, during the trip that he took with his son, according to ProPublica.
Andy Surabian, a spokesperson for Mr Trump Jr, did not answer a question about what the pair discussed.
Mr Surabian said Mr Trump Jr bought the trip to Mongolia at a National Rifle Association (NRA) auction in 2015, before his father announced his candidacy for president.
He used his own funds to pay for the trip, flew commercial and got the required permits through a third-party outfitter, Mr Surabian said. He said neither US nor Mongolian officials helped to organise Mr Trump Jr’s trip.
Jandos Kontorbai Ahat, a member of the Mongolian president’s political party, arranged the hunting trip, ProPublica said. He said the trophy hunting system in Mongolia is “very political”.
He said the US Embassy in Mongolia’s defence attache accompanied Mr Trump Jr and the other hunters.
Hunting guides and scouts said that five American bodyguards also protected Mr Trump Jr. His spokesperson said the Secret Service, and not Mr Trump Jr, determines what security protocols are necessary.
The president’s son documented his trip in Instagram posts this autumn in which he stood in front of a yurt, rode a horse and held a live eagle.
Mr Ahat said Mr Trump Jr was “an upstanding person” who treated others with respect. The local guides said his hunting skills impressed them.
Mr Trump Jr and his brother Eric Trump are avid big-game hunters who have killed animals on African safaris before. During then-candidate Mr Trump’s presidential campaign, photos resurfaced of his sons posing with an elephant, a buffalo and a leopard they had killed on a safari.
In February, a hunting advocacy and lobbying website raffled off a five-day elk hunt in Utah with Mr Trump Jr.
The Washington Post
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