WASHINGTON -- A forensic scientist testified Friday that Roger Clemens' DNA was found on a needle and two cotton balls that former strength-and-conditioning coach Brian McNamee says he kept for several years before turning them over to the government, introducing a crucial element of the prosecution's case in the federal perjury trial of the former pitcher.

But the testimony from Alan Keel, the DNA technical leader of Forensic Research Associates, was met with rigorous cross-examination by the defense and several skeptical questions submitted by jurors as he completed a full day on the stand.

At the end of a day that was projected as the one on which the prosecution would rest its case -- but fell short of doing so -- Judge Reggie Walton announced that one of the 14 remaining jurors had just been informed that her mother passed away. Walton said it was likely but not certain that the jury pool might be down to 13 -- 12 jurors and one alternate -- when court reconvenes on Tuesday following the Memorial Day holiday weekend, depending on the condition of the juror, a Metro transit police officer.

The testimony marked the first time Clemens' DNA, or genetic fingerprint, has been linked to items associated with the use of performance-enhancing drugs. McNamee said he kept the evidence after injecting Clemens with steroids in August 2001, storing the items in a beer can and a mailing box until handing them over to authorities in January 2008. Actually, several of the items, including bloody gauze and tissues, were found with blood determined to possess McNamee's DNA.

In cross, defense attorney Michael Attanasio pointed out that the 1-in-449 random probability of the DNA on the needle being associated with Clemens was vastly less definitive than other items Keel analyzed, which was one in tens of millions or more. Also, many queries among the more than 20 submitted by jurors questioned the validity of some of Keel's findings and mentioned several of the defense's theories about the evidence being contaminated.

As established through Keel's testimony, blood on one cotton ball, pus on another cotton ball and cells on a needle with a gray hub on it were determined to be consistent with Clemens' DNA.

"It is compatible with Mr. Clemens," Keel said with each item that included Clemens' DNA.

The needle identified with Clemens was not stored inside the beer can, but the cotton balls were, according to Keel's testimony and previous exhibits in the trial. On Thursday, a scientist entered as an expert witness testified that steroids were found on the needle with the gray hub and on cotton balls, although those associations have not yet been established by testimony.

Under direct examination from Assistant U.S. Attorney Courtney Saleski, Keel said the blood on the one cotton ball could be associated with only one in 15.4 trillion U.S. Caucasians. The pus on the other cotton ball could be associated with only one in 173 trillion U.S. Caucasians. Essentially, they were determined to be unique to Clemens.

"I would expect this profile to be unique to only one person who has ever lived on the planet," Keel said about each of those two matching profiles.

The needle, however, had only "six to 12" white blood cells that were available to be tested, and Keel testified that the DNA profile was consistent with one out of 449 U.S. Caucasians. Keel added that "everything is consistent with originating from Mr. Clemens."

During cross, Attanasio leaped on the difference in numbers relating to the probability it was Clemens' DNA on the needle, pointing out that Keel testified on direct that the match to one area that included McNamee's DNA on the gauze was 1 in 1.8 quintillion.

Attanasio said of the 1-in-449 number relating to the needle, "That's a pretty weak statistic in your field, isn't it?"

"No, it's not a weak statistic at all," Keel responded, saying that "if you're not the source, you're going to be eliminated" from being associated with that item.

Attanasio, in a query that was echoed by a juror, questioned why Keel didn't analyze certain items that were submitted to him. Keel said on direct that he didn't find substantial enough biological material on those items. Attanasio asked about an item that had possible biological material on it but Keel didn't test, saying, "You didn't want to know?"

Attanasio also suggested that the drops of blood on one of the cotton balls -- which on direct Keel said would be consistent to dabbing an injection site -- could have come from other things, such as a shaving cut or a blood blister, or from dabbing a "lawful injection" of vitamin B12.

"It could come from dozens of things as long as there's a small amount of blood, right?" Attanasio said. "Yes," Keel said.

As for the cotton ball with pus on it, Attanasio attempted to make it seem impossible it could come from an injection because pus indicates an infection is present.

"It would be inconsistent with your analysis that these items with yellow stains of pus came from the act of injection at the time of injection, correct?" Attanasio said. "That's correct," Keel said.

On direct, Keel testified that a drop of blood has something in the order of 20,000 to 30,000 white blood cells, whereas the sample on the needle had just the six to 12. With that in mind, Saleski asked whether those cells could somehow be planted on the needle, attempting to head off the defense's anticipated suggestion that McNamee manufactured the evidence.

"If this was contrived, you'd expect to find much more biological material," Keel replied and maintained through cross.

Keel, an experienced DNA expert based in Hayward, Calif., whose bachelor's degree is in zoology, testified Thursday that he had done DNA analysis in hundreds of cases, estimating that about 60 percent of the cases have been for prosecutors and 40 percent for defendants. He said the testing was done blindly, and he only found out the identities of the persons involved in recent days.

Still, one juror question asked why he should be considered an expert -- Walton said he will explain the legal aspect to that in jury instructions -- another asked how the codes (Clemens was No. 1 and McNamee No. 2) could be so transparent as to the principals in the case and another asked how he could be sure that there was no contamination on the items he analyzed. It remains to be seen whether those provide an indication about which way the jury is leaning or it's just due diligence on jurors' part.

Clemens is charged with three counts of making false statements, two counts of perjury and one count of obstruction of Congress based on his testimony during a Feb. 13, 2008, hearing before the House Committee on Oversight and Government Reform and a Feb. 5, 2008, deposition conducted by committee staff members. Clemens said at the hearing, "Let me be clear: I have never used steroids or HGH." Clemens testified that McNamee only ever injected him with vitamin B12 and lidocaine.

McNamee, who served as a strength-and-conditioning trainer to Clemens in one capacity or another for nearly a decade, said in his own deposition and at that same hearing that he had injected Clemens with performance-enhancing drugs on numerous occasions, keeping items he says proves it.

Another pending development in the case is a motion to quash a subpoena issued by the Clemens team to have Rep. Darrell Issa (R-Calif.) testify. Issa is currently the chairman of the House Committee on Oversight and Government Reform, and was a member of the committee in 2008 and in fact sat in on some of Clemens' deposition. Walton said he was considering denying the motion to quash because Issa had made public statements about the trial, but also expressed doubt that Issa would in fact appear because he is protected from testifying in courts other than Congress itself by the Speech or Debate clause in the U.S. Constitution.

Friday was the 23rd day of proceedings and completed the sixth week since jury selection began April 16. Keel was the 21st witness called by the prosecution, which has said two more will be called before resting its case.