A 22-year-old Haymarket man who pleaded guilty to second-degree murder for helping his friends kill a Loudoun County scientist yesterday fired his attorneys and said he wants to withdraw his plea.

Michael P. Pfohl appeared in Loudoun Circuit Court for what was expected to be a routine sentencing hearing after he had entered a plea that exposed him to as much as 21 years and four months in prison for his role in the 2001 slaying of Robert Schwartz.

Schwartz, 57, was slashed and stabbed with a sword in his Loudoun farmhouse. Pfohl admitted in court in December that he knew his friend Kyle Hulbert planned to kill Schwartz and that he agreed to drive Hulbert to Schwartz's house.

Pfohl did not say yesterday why he will argue that he should be allowed to back out of his plea, but he told Judge Burke F. McCahill that he had hired a new lawyer. That lawyer, Spencer Ault, said he would file a detailed motion early next week, and McCahill scheduled a May 5 hearing.

Pfohl apologized to the judge for "any inconvenience." "I also apologize to the Schwartz family for the shock and disappointment this might cause," he said.

Commonwealth's Attorney Robert D. Anderson said that he would oppose Ault's motion but that he was prepared to try Pfohl on the more serious first-degree murder charge.

Robert Schwartz's youngest daughter, Clara Jane Schwartz, 20, was convicted of orchestrating her father's killing, and three of her friends, including Pfohl, were accused of involvement in the plot. The friends shared an interest in witchcraft and fantasy worlds.

On Dec. 8, 2001, Pfohl and his girlfriend, Katherine Inglis, 20, drove with Hulbert to Robert Schwartz's remote Leesburg house, prosecutors said. Hulbert, 19, who has pleaded guilty to murder, went inside alone and killed Schwartz. Inglis is awaiting trial on a conspiracy charge.

Legal experts said it is difficult but not impossible to withdraw a plea before sentencing and that the judge has wide discretion. A judge will generally review the transcript of the plea hearing, during which defendants are asked a series of questions to determine whether they understand the agreement.

"What the judge will look at is whether the plea was freely and voluntarily and intelligently entered into by the defendant and did he know the consequences of his actions," said Alexandria Commonwealth's Attorney S. Randolph Sengel, who is not involved in the case.

Pfohl's mother, Beth Pfohl, said her son felt pressured by his former attorney, Jensen Barber, to plead guilty. Barber said he believes the motion to withdraw is not in Michael Pfohl's best interest and said it had been spurred by Beth Pfohl.

"At every junction in this case in which Ms. Pfohl can insert herself to make things worse for her son, she has done so," Barber said in court. If convicted of first-degree murder, Pfohl would face life in prison.

During Pfohl's Dec. 20 plea hearing, McCahill asked him whether he was guilty of murder. "As much as I hate to say it, yes," Pfohl replied.

But Beth Pfohl said that her son did not grasp Hulbert's intent. "Kyle made fantastic claims all the time," she said.

In a statement to police made shortly after his arrest, Pfohl said he "drove Kyle to Clara's house . . . to assassinate someone."

"I knew what I was getting myself into, even if only vaguely," he wrote.

Michael P. Pfohl felt pressured by his former attorney to plead guilty, Pfohl's mother said.