A visitor to Yellowstone National Park may not have realized she was staring death in the face when she approached a bison lounging out on the grass over the weekend, and reached out to pet the giant animal.

The woman walked away unhurt -- but she could still get hit with a fine, park officials told KRTV on Tuesday. They say the animals can attack without warning.

Bison have killed three people in the park's history. The beasts gored at least five people last year.

A witness recorded the daring encounter on Saturday -- Yellowstone's opening day for visitors -- and posted the video online.

Other people can be heard reacting with shock and surprise. One witness says, "She's totally antagonizing it. Where are the rangers?"

The bison appeared to be sleepy. At one point, the animal jerked up its head, hitting the woman in her hand as she stroked its fur.

Seconds later, she walked off. The video showed one other person, possibly a companion, also recording her from a safer distance.

Officials have not identified the woman. "When you harass wildlife, you could face a citation or a mandatory appearance in front of a federal magistrate," park spokeswoman Amy Bartlett told KRTV.

Last year, a Yellowstone bison hurled a 43-year-old Mississippi woman in the air as she posed for a selfie. In other incidents, bison gored a 68-year-old woman and a 16-year-old girl and tossed an off-trail teenager and an Australian tourist into the air.

As for the latest encounter, Bartlett said, "This woman is extremely lucky. We can’t explain why he didn’t get up."

The Associated Press contributed to this report.