Alex Yusik, a board-certified forensic psychologist, explained the difference between serial killers, mass murderers and spree killers.
Serial killer: Three or more people being killed; there is usually a particular pattern with ritualistic crime scenes. Example: Jack the Ripper cut victims' throats before mutilating their abdomens.
Mass murderer: One individual perpetrates a large amount of killings in a short amount of time. Example: The Jim Jones Massacre, in which 909 inhabitants died of cyanide poisoning.
Spree killer: When someone goes "postal," like a disgruntled employee. In a short time, a large number of people are killed. Example: Columbine shootings, in which two students killed 12 people and wounded 23 others before taking their own lives.
Former medical student Philip Markoff, 22, was accused of killing women he met through Craigslist.
His alleged crimes include what a prosecutor described as the "brutal murder" of Julissa Brisman, a 26-year-old masseuse who fought back against her attacker.
Brisman was bashed in the head and shot three times inside the Marriott Copley Hotel, with the fatal bullet piercing her heart.
Markoff was then charged with fatally shooting Brisman on April 14, 2009, and with the armed robbery and kidnapping of Trisha Leffler, 29, a prostitute who was tied up at the Westin Copley on April 10.
Brisman had advertised her masseuse service on Craigslist; Leffler had also advertised through the website.
On Aug. 15, 2010, Markoff killed himself inside Boston's Nashua Street Jail, where he was being held in custody awaiting trial.