Four U.S. senators are banding together to ask Secretary of the Interior Sally Jewell to speed up her review of an Endangered Species Act rule they hope will help protect and preserve the African lion.

Senators Bob Menendez and Cory Booker, both D-N.J., Richard Blumenthal, D-Conn., and Edward Markey, D-Mass., all signed off on a letter addressed to Jewell where they noted, "The United States is the world's largest importer of African lion parts as hunting trophies and for commercial purposes."

They added listing the African lion as "threatened" would pave the way for the U.S. Fish and Wildlife Service department to set a standard that only allows for the import of sport-hunted lion trophies from countries with sound conservation plans "that enhance the survival of the species in the wild."

The senators' actions come in the wake of an American hunter recently traveling thousands of miles abroad to track and target the famed African lion Cecil for death, all in the name of sport and trophy hunting.

In addition to requesting a complete investigation to determine if any domestic or international rules were broken by the hunter's actions, the letter also pointed out that Cecil is only one of hundreds of African lions similarly targeted each year.

According to the letter, it's estimated that over 400 lions are now lost to American trophy hunters each year. Over the last thirty years, the number of African lions has declined by almost 50 percent, with fewer than 40,000 now existing overall.

CNN reports Cecil's killer, Dr. Walter Palmer, allegedly paid around $50,000 for the right to target and track him. The 13-year-old lion was lured out of his surroundings with food, shot with a crossbow, tracked for 40 more hours and then finished off with a gun.