CHEYENNE - The photos showed mangled and bloody antelope carcasses, a man giving a thumbs-up next to the blood-splattered grille of a pickup truck, and "HA HA!!!" written over the image of a severed antelope head.

"We sped up. Most of the herd got out of the way," someone wrote in a posting on an off-roading Internet discussion board that included the grisly photos.

Now, authorities say they have arrested a 23-year-old gas field worker from Rock Springs after tracing the license plate of the truck in the photo.

"Anyone who has any regard for wildlife in this case would absolutely be shocked," said Scott Talbott, a wildlife specialist with the state Game and Fish Department.

The gas field worker, Jonathan Hefner, pleaded not guilty Wednesday to two counts of wanton destruction of big game, a misdemeanor punishable by up to a year in jail and a $10,000 fine for each count. A telephone message left for his attorney, Harold Moneyhun, was not immediately returned Thursday.

State authorities began investigating after receiving an anonymous tip on the state's "Stop Poaching" Web site.

Talbott said wildlife investigators recognized the area from the photos, dated Dec. 7, and had no trouble finding where the antelope were killed. DNA samples were taken from antelope parts found along the dirt road.

Talbott said investigators determined Hefner worked for National Oilwell Varco and was driving a company truck when he deliberately drove into the herd of about 20 antelope on a remote stretch near Farson.

A person who answered the phone at the National Oilwell Varco office in Rock Springs said Hefner no longer worked there. A message left for a company spokeswoman in Houston was not immediately returned.

The posting, dated Saturday, was taken down by Thursday. Copies of the posting were provided to The Associated Press by the governor's office.

In the posting, someone named "BigRed350x" described what happened: "So here we are cruising down the road, jon (bonbon) driving and me sleeping in the passenger seat when jon wakes me. … I look ahead and there's a herd of about 20 antelope just standing in the road. … we sped up. Most of the herd got out of the way, but two … dumber ones turned back for the greener grass on the other side of the road. LOL Well, they didn't make it."

Although Hefner was the only person arrested, Sweetwater County Attorney Brett Johnson said others may still be charged.

"Wyoming takes its big game seriously, and our office certainly takes this case seriously," Johnson said.