WASHINGTON — Lawmakers on the congressional committee investigating steroid use in baseball will have two key questions to answer as they pore over more than four hours of contradictory and occasionally combative testimony given yesterday by Roger Clemens and his accuser, Brian McNamee: Which man lied, and should he be prosecuted?

Clemens, appearing here at a nationally televised hearing, forcefully denied ever having used performance-enhancing drugs and explicitly contradicted assertions about his alleged use of Human Growth Hormone made by his close friend and training partner, pitcher Andy Pettitte.

“Let me be clear: I have never taken steroids or HGH,” Clemens said at the outset of the hearing, his eyes glaring in the direction of McNamee, the former trainer who testified that he injected the seven-time Cy Young award winner with illegal drugs on “numerous occasions.”

Clemens and McNamee were seated several feet apart at a witness table yesterday on Capitol Hill, separated only by Charles Scheeler, a lawyer who helped to author the 409-page report published by a former Maine senator, George Mitchell, last year. The report relied heavily on accusations from McNamee and has cast a shadow over Clemens, whose ticket to baseball’s Hall of Fame had been all but written.

Under intense questioning from lawmakers, neither man deviated from the opposing stories they have offered for weeks, but both admitted to inconsistencies and, in McNamee’s case, outright lies in their accounts that have damaged their credibility.

After the hearing, committee leaders said it was clear one man was not telling the truth, but they said they were unsure if they would request a perjury investigation by the Department of Justice. “I haven’t reached any conclusions at this point,” the Democratic chairman of the House Committee on Oversight and Government Reform, Rep. Henry Waxman of California, told reporters. “Someone lying is not necessarily committing perjury,” he added. Federal prosecutors, who Waxman said are in possession of physical evidence allegedly implicating Clemens, could ignore a request from the committee or decide to open an investigation on their own.

By testifying under oath yesterday, Clemens knowingly risked charges of perjury that could throw a final dagger through his legacy as one of baseball’s most dominating pitchers in the last quarter-century. Known for his intensity on the mound, the hard-throwing hurler racked up 354 victories and 4,672 strikeouts over 24 seasons with the Red Sox, Blue Jays, Astros, and Yankees. He likely pitched his final game in pinstripes last fall.

He acknowledged the uphill battle he faces in his opening statement, expressing frustration over his circumstances. “How in the world can I prove a negative?” Clemens said. “No matter what we discuss here today, I am never going to have my name restored.”

By far his most difficult task yesterday was to rebut the sworn statements provided by Pettitte, who largely corroborated McNamee’s allegations and asserted that Clemens had told him directly that he had used human growth hormone.

With Rep. Elijah Cummings, a Democrat of Maryland, repeatedly reminding him that he was under oath, Clemens testified that Pettitte had “misremembered” the conversation in which Pettitte said Clemens had admitted his HGH use. He said he only told Pettitte about seeing three older men on television praising the drug, not that he actually used it. Pettitte has admitted to taking HGH on two occasions, which Clemens said Pettitte never told him about. “I was shocked. I had no idea,” Clemens said of Pettitte’s use of HGH.

Cummings pointed out the Pettitte’s wife, Laura, had corroborated his account in a sworn affidavit.

While he has disparaged McNamee repeatedly since the Mitchell report came out in December and has filed a lawsuit against him, Clemens had only kind words for Pettitte, with whom he has trained during the offseason. He called him a “fine gentleman” and “a very honest fellow” at various points during his testimony. “Andy Pettitte is my friend,” Clemens said. “He was my friend before this, he will be my friend after this.”

When Cummings returned to question Clemens again hours later, he was unmoved. “It’s hard to believe you, sir,” he said. “I hate to say it. You’re one of my heroes, but it’s hard to believe you.”

With Clemens and McNamee unwilling to budge from their accounts, the hearing was ultimately a spectacle. Some lawmakers clearly took sides against one of the men, while others said they simply did not know who to believe.

The drama peaked when Mr. Waxman castigated Clemens and suggested he had tampered with a committee witness by bringing a former nanny to his Houston home last weekend and allowing his own attorneys to interview her before she spoke to congressional investigators.

“I don’t know if there was anything improper in this,” Waxman said. “I do know it sure raises a perception of impropriety. The impression it leaves is terrible.”

The nanny, whose name was not released, told the committee that Clemens had attended a 1998 party at the home of Jose Canseco in which McNamee alleges Clemens talked to him about steroids. Clemens said he did not attend the party and was playing golf that day.

Waxman asked Clemens if it was his or his attorneys’ idea to contact the nanny before giving her name to the committee, a query that prompted Clemens’s lawyers to jump up from their seats behind him and interrupt the hearing.

“It was my idea to investigate what witnesses know — just like any other lawyer in the free world does,” a Clemens attorney, Rusty Hardin shouted, as he stood with his hand on the pitcher’s shoulder. Waxman then ordered him silent, saying lawyers were not permitted to speak at the hearing.

Clemens said he was merely trying to help put the committee in touch with his former nanny, whom he said he had not seen in several years. “Mr. Chairman, I was doing y’all a favor,” he said.

Waxman seemed to favor McNamee, saying after the hearing that he found him “credible” and noting that his account was supported by Pettitte and Chuck Knoblauch, another former Yankees player. Both athletes were excused from yesterday’s hearing to prevent them from having to implicate Clemens face-to-face.

McNamee got far less sympathy from other lawmakers, most notably Rep. Daniel Burton, an Indiana Republican.

Burton made McNamee recount numerous times that he had lied about his involvement with HGH and steroids before he cooperated with the Mitchell investigations. He was also incredulous that McNamee had kept for more than seven years syringes and bloody gauze pads that he is now presenting as physical evidence of Clemens’s steroid use.

“Why didn’t you give them to the Mitchell report?” Burton asked at one point.

“I felt horrible to be in the position I was in,” McNamee said.

“Gee whiz? Are you kidding me?” Burton exclaimed. He added moments later: “I know one thing I don’t believe, and that’s you.”

Another Republican lawmaker, Rep. Christopher Shays of Connecticut, told the former trainer point blank: “Mr. McNamee, you are a drug dealer.”