DEDHAM -- Accused killer Thomas Lally was obsessed with television shows featuring forensic science to learn how to fool investigators and get away with murder, a prosecutor said Friday at the opening of his trial.

`` He became very knowledgeable how to commit crimes, how to cover up crimes. He became very obsessed with these shows,'' Assistant District Attorney Susan Corcoran said in Dedham Superior Court.

Lally, 24, of Norton, is the alleged mastermind in a plot with two friends to kill Marina Calabro, the 84-year-old great aunt of his friend and codefendant, Anthony Calabro.

Lally allegedly hit Marina Calabro over the head with a cast iron frying pan and a tea kettle on Dec. 19, 2001, while his friend Jason Weir watched and Calabro remain outside as the lookout.

He later placed her body at the bottom of a flight of stairs to make it appear that the retired hair dresser had fallen accidentally to her death, the prosecutor said. The frying pan, which broke from the force of the blow, and the tea kettle were found at Meadow Brook Pond in Norton not far from Lally's Taunton Avenue home, Corcoran said.

The men, described as freeloading high school dropouts at the time, wanted to share in the pro ceeds of Anthony Calabro's $260,000 inheritance and Marina Calabro's three-decker apartment house in Quincy, where she had lived all her life.

Anthony Calabro, 22, will be tried separately at a later date.

Weir, 20, has agreed to testify against Lally in an agreement with prosecutors to plead guilty to manslaughter and receive a 10-year prison sentence. A murder conviction carries a life sentence.

While the prosecution depicted Lally as a maniacal killer of a helpless elderly woman, Lally's lawyer, Robert Griffin, said the prosecution nailed the wrong man for the crime.

Lally was scheduled to receive more than $80,000 in annuities over 10 years and had nothing to gain from killing Calabro, Griffin said.

Weir had no money, he said, and lived from house to house in Norton.

`` Jason Weir is the person who had the motive to do this. Jason Weir is the one who had the opportunity to do this,'' Griffin told the 16-member jury.

Referring to Weir's agreement with the prosecution, Griffin told the jury that Weir `` will tell you anything he thinks the government wants you to hear.''

He said Weir later bragged about the killing to a friend and threatened Lally and his family, who once took him in when his mother kicked him out of the house. The defense lawyer said the case amounted to a `` series of huge mistakes'' by police investigators, who he said bungled the DNA evidence and blew the investigation.

During her opening statement, Corcoran acknowledged that a crime lab technician contaminated some DNA evidence, but urged the jury not to be clouded by a single piece of evidence.

She said other DNA evidence collected from the fingernails of the victim failed to exclude Lally, while excluding Weir and Calabro.

Lally was questioned by police the night of the murder about scratches on his face, and he told them he got them during a fight the previous night with Anthony Calabro, Corcoran said.

The prosecutor detailed for the jury how Marina Calabro fought for her life and screamed for help before Lally grabbed her neck and told her, `` Let go. Just go. Anthony wants it this way.''

The jury was taken to Meadow Brook Pond in Norton to see where the alleged murder weapons were disposed of by Calabro and Lally and the Quincy apartment house where the murder occurred.

The trial is scheduled to resume Monday with testimony of police officers and the medical examiner, who initially ruled the death a homicide before police learned about the plot to kill Calabro.