LONDON (AP) — The former head of the World Anti-Doping Agency says Maria Sharapova was guilty of "willful negligence" for using meldonium, and international tennis officials knew that many players were taking the drug before it was banned this year.

Pound tells The Associated Press that Sharapova could face a ban of up to four years unless she can prove mitigating circumstances to explain her positive test for meldonium at the Australian Open in January.

Meldonium, a Latvian-manufactured drug designed to treat heart conditions, was added to the World Anti-Doping Agency's banned list on Jan. 1 after authorities noticed widespread use of the substances among athletes.

Sharapova said she had been using the drug for 10 years for various medical issues. She said she didn't realize it had been banned this year.

Pound says "it was willful negligence to miss that."