Howard Jacobs said Taurasi's "A" sample came back positive from a Turkish lab last week and she was provisionally suspended by the Turkish league she's playing in. She has already missed three games with her team, Fenerbahce.
Jacobs said the substance "was not a steroid or recreational drug," and that Taurasi has asked that her "B" sample be tested.
"We're taking it one step at a time," Jacobs said. "I'd rather not say what it is at this stage, they've only tested the 'A' sample. Somehow it leaked over in Turkey. We're waiting for the 'B' sample to be tested. We've had some difficulty with getting a date so far, mostly because of the holidays. We're hoping to get it as soon as we can."
Taurasi's positive test came to light two days after her former Huskies won their 89th straight game, surpassing the UCLA's men's team winning streak set from 1971-74. Taurasi helped lead UConn to three straight national championships as well as 70 consecutive victories from 2001-03.
"While she is fully cooperating with authorities, there are serious doubts about the accuracy of the test results," Jacobs said in a statement to the AP. "We are confident that Diana will be fully vindicated once all the evidence is reviewed. She regrets that someone has violated the confidentiality rules of this process, and will make no further statement at this time."
WNBA spokesman Ron Howard said the league had no comment.
Jacobs said: "In the 10 years of competition at the collegiate, professional and Olympic level, Diana Taurasi has never taken, been suspected of using, or tested positive for any performance enhancing substance."
Taurasi led the WNBA in scoring for a league-record fourth straight year, getting 22.6 points per game. The five-time All-Star and two-time WNBA champion signed a multiyear contract extension with the Phoenix Mercury in August.
She served one day in jail and was suspended by the WNBA for two games in 2009 after pleading guilty to a DUI charge.