Prince George's County prosecutors this week dropped murder charges against a Landover man, marking the 12th time in 14 months that they have dropped charges against defendants accused of homicide or other violent crimes.

Prosecutors also abandoned charges last month against a man accused of the attempted murder of four people, three of whom were shot and wounded.

As in most of the dismissed cases over the 14-month period, prosecutors said key state witnesses could not be found or were otherwise unavailable when the trials were to begin. Police and prosecutors have said they think witnesses in Prince George's often disappear or stop cooperating because they fear retaliation.

In the most recent case, prosecutors knew where a key state witness was Tuesday morning, but could not get him to court in time, said State's Attorney Glenn F. Ivey.

Sixty-five potential jurors were in the Upper Marlboro courtroom, prepared for jury selection to begin in the scheduled murder trial of Rashard D. Hubbard, 26, of Landover.

Before the selection process began, Assistant State's Attorney Clayton Aarons told Circuit Court Judge. C. Philip Nichols Jr. that the state was dropping charges because of insufficient evidence.

Hubbard was charged with the Oct. 12, 2005, murder of Lamont D. Avent. At about 10 p.m. that night, police found Avent on a playground at a Landover townhouse complex, suffering from gunshot wounds. Avent was taken to Prince George's Hospital Center, where he was pronounced dead.

According to court records, one witness told police that he saw Hubbard, limping from a gunshot wound to the leg, holding two handguns, a .38-caliber revolver and a .45-caliber semiautomatic.

Firearms examinations of the two handguns showed that bullets recovered from Avent's body were a "match" to the two guns Hubbard had, court records say.

Weeks before Hubbard's trial, the witness who saw Hubbard with the two handguns couldn't be found, Ivey said. The witness failed to appear for a November hearing in Prince George's Circuit Court in which he was to be sentenced on a cocaine possession charge, Ivey said.

Authorities in Philadelphia picked up the witness last week, Ivey said. But he will not have an extradition hearing until March 2, past the deadline prosecutors had under state law to bring Hubbard to trial.

Before the charges were dropped, Hubbard turned down a deal that called for him to plead guilty to manslaughter and be sentenced to five years in prison, said defense attorney Antoini M. Jones. "I didn't think they had a strong case," Jones said.

In the other recent case, Assistant State's Attorney Joseph Wright dropped attempted murder charges against Samuel R. Goodwin, 20, of Fredericksburg, Va., on Jan. 24, the day his trial was to start.

Goodwin was charged with trying to kill four people June 6, 2006, in Oxon Hill; three of the victims were shot and wounded. Goodwin handed the handgun used in the shootings to the man who fired the shots, according to court records.

Thomas C. Mooney, Goodwin's defense attorney, said the charges were dropped because state witnesses did not appear for trial.