Belmont Stakes contender Charlatan, one of trainer Bob Baffert's horses, tested positive for lidocaine on May 2, according to multiple reports.
The Louisville Courier-Journal backed reports from the New York Times and Bloodhorse.com that two Baffert horses were among three positive tests at Arkansas' Oaklawn Park.
Charlatan is ranked fourth in Kentucky Derby points. However, the positive test could result in a disqualification from his win in the Arkansas Derby on May 2, which could affect his eligibility for the Kentucky Derby.
Undefeated Nadal, listed as the current Kentucky Derby favorite by TVG.com, is also trained by Baffert but didn't test positive.
"The rules of the Arkansas Racing Commission mandate confidentiality concerning any investigation into an alleged rule violation until there is a written decision of the Stewards," Baffert said in a statement. "I am extremely disappointed that, in this instance, the Commission has not followed its own rules on confidentiality.
"I am hoping for an expedited investigation and look forward to being able to speak soon about any written decision of the Stewards, if and when it becomes necessary and I'm allowed to under the Commission's confidentiality rules."
The second Baffert horse who tested positive for lidocaine, a regulated anesthetic widely used in equine medicine, is 3-year-old filly Gamine.
Baffert requested a second round of testing on the horse's samples.
Rescheduled for June 20, the Belmont Stakes will be the first leg of the Triple Crown in 2020.
The Kentucky Derby, originally scheduled for May 2, was moved to Labor Day weekend in September due to the coronavirus pandemic. The Preakness was pushed back from May 16 to Oct. 3.
Nadal, trained by the two-time Triple Crown winner Baffert, emerged as the Derby favorite after winning the Grade 2 San Vicente Stakes at Santa Anita.
--Field Level Media