Four other drivers had no drugs in their system, according to Nixon's memo, a copy of which was obtained by The Salt Lake Tribune.
Yet in every case, Steed wrote reports claiming the drivers showed signs of impairment, such as dilated pupils and leg and body tremors.
Nixon referred to "a pattern" of conflicting information between Steed's arrest reports and the laboratory results.
"This is something that needs to be addressed before defense attorneys catch on and her credibility along with the DUI squad's credibility is compromised," Nixon wrote.
A UHP spokesman declined to comment on the memo Wednesday.
"We're considering that a protected document so we're not at liberty to discuss the document," said Dwayne Baird, the spokesman.
Greg Skordas, an attorney representing Steed, said his client was never made aware of Nixon's memo, nor did her superiors speak with Steed about the specific issues raised in the memo.
"She doesn't know what is being referred to here," Skordas said.
The memo is "extraordinary," said Joseph Jardine, a Salt Lake City defense attorney who earlier this year successfully petitioned a judge to gain access to Steed's discipline record. Jardine said the memo was not included in the records he viewed. The Salt Lake Tribune showed Jardine and Skordas the memo Tuesday.
Jardine said the memo shows the UHP hid Steed's problems from defense lawyers. Legal precedent requires prosecutors also to have been aware of the memo and disclose it to defendants, Jardine said.
"That's crazy," Jardine said of withholding the memo. "That's exactly contrary to what the law requires them to do."
Steed's credibility was compromised in April. One state judge in Salt Lake County and one in Davis County found Steed had been untruthful on the witness stand during DUI and drug possession cases. The rulings led to a dismissal of charges against both defendants.
Soon after the rulings, the UHP removed Steed from road duty and began its own investigation of Steed. It's unclear when that investigation will be completed. Skordas said Steed has been reassigned to an administrative job out of uniform and without a gun while the investigation is under way.
Nixon's memo was dated May 14, 2010, and addressed to Lt. Steve Winward, who was then head of the UHP DUI Squad. Steed was assigned to the squad at that time, too.
Nixon wrote that he began his review after a prosecutor declined to prosecute one of Steed's arrests in Salt Lake City Justice Court. The suspect tested negative for drugs, yet Steed reported the suspect showed multiple signs of impairment.
The 20 cases Nixon reviewed were from 2009. Nixon reviewed a report from Steed about an arrest in which he assisted two days before he wrote his memo. Nixon found Steed's report contradicted with what he observed at the scene.
"I did not have a pupilometer, however the eyes were obviously on the low end of normal," Nixon wrote of that arrest. "There was no mistaking them for dilated. I went over the report on that tonight when I got in my office. The report shows that the suspect had dilated pupils. It also showed that his hand were [sic] moving uncontrollably."
But Nixon noted that the suspect "was able to sit calmly while having his blood drawn."
In every case he reviewed, Nixon wrote, Steed claimed the suspect had dilated pupils. Jardine said a check of pupils is a standard part of a DUI field exam and something that cannot be corroborated by a lab test or dashboard video.
"It's a clever lie because nobody can verify it," Jardine said.
Nixon's memo doesn't say how many of the 20 cases resulted in convictions.
It's not known what Winward or his superiors did after receiving Nixon's memo. UHP has disclosed documents showing Nixon issued two reprimands to Steed earlier in 2010, but there's no indication Steed was disciplined for the questionable DUI drug arrests.
The reprimands issued by Nixon were for Steed turning off her microphone during a traffic stop where she issued sobriety tests out of sequence and for taking a blood sample from a suspect on the side of the road without waiting for another trooper to assist her.
Steed was the UHP trooper of the year in 2007.