CODY, Wyo. (AP) — Yellowstone National Park officials have asked 2017 visitors to adopt its "Safe Selfie" policy.

Yellowstone officials have issued the policy in response to a rash of irresponsible and illegal behavior the past two years.

Yellowstone broke records by topping 4 million tourists for the first time in 2015 and then topped that with 4.2 million visitors last year. Mixed in the millions, a number of visitors have made bad decisions.

Two summers ago, five people taking selfie photographs provoked bison into attacking them. No one was killed, but medical treatment had been needed.

Last year, a tourist had died after walking off a pedestrian boardwalk into a thermal area.

The Yellowstone East Gate opened at 8 a.m. Friday.