In his closing argument, Patrick Haggan said victim Io Nachtwey was terrified as they drove to the bridge because she knew the men planned to kill the homeless woman. She first tried to jump from the moving car and then agreed to have sex with the gang leader in the desperate hope he'd protect her, he said.

But the three men had already decided to murder her to send a message to a rebellious faction of their fledging gang, Haggan said. They got a drug addict to do the actual stabbing by threatening to kill him, as well, he said.

"This was a cowardly, vicious and senseless murder and each and every one of the defendants wanted it to happen or chose to allow it to happen," Haggan said.

"The time for accountability is now," he said.

The jury was expected to begin deliberations tomorrow morning after more than six weeks of testimony, said David Procopio, a spokesman for the Suffolk County District Attorney's Office.

Ismael Vasquez, 27; his brother, Luis Vasquez, 23; Harold Parker, 31; and Scott Davenport, 31, are all charged with first-degree murder in the death of Nachtwey, a native of Hawai'i who hung out in a section of Harvard Square called "The Pit" with other young drifters and homeless, some of whom the gang leaders tried to recruit to steal for them.

Prosecutors said Davenport stabbed Nachtwey a dozen times, then Luis Vasquez cracked nunchucks on her skull to silence her screaming. They then allegedly rolled her body into the Charles River.

Haggan said Nachtwey was chosen for execution in part because her boyfriend led a revolt of the gang recruits, who refused to steal after the boyfriend questioned whether the gang leaders were frauds, and also because she was left behind when the others were sent out to steal.

"They took the easy target, the easy prey," Haggan said.

Earlier today, Davenport's lawyer asked the jury for a "merciful verdict" of innocent for his client. In his closing, John Courtney said Davenport hooked up with the others earlier only because he knew they could get him heroin, then drove with them to Boston because he was too high to take his car himself.

Davenport didn't even know Nachtwey when Ismael Vasquez and Parker said they'd kill him if he didn't stab Nachtwey, Courtney said.

"Scott Davenport was put in the position of having to decide whether he lived or he died," Courtney said. "They took Scott Davenport and said, 'You're going to kill for us."'

But Parker's lawyer said Davenport was trying to deflect the jury's attention from the fact that he was Nachtwey's killer.

"Scott Davenport had the audacity to tell you he was the victim here," said attorney Jonathan Shapiro. "He wants you to take pity on him. He wants you to let him get away with murder."

During closing arguments, defense attorneys also attacked the two key prosecution witnesses, Ana White and Lauren Alleyne, both 21, who avoided life in prison when they pleaded guilty to helping to kill Nachtwey by holding her down as Davenport stabbed her. In exchange, they testified against the four men.

Shapiro said the case against his client came down to their testimony, which he said was inconsistent and unbelievable, concocted only to save their own necks.

"The stakes are so high for these admitted liars that you cannot believe a word that they say," he said.