Two cousins have been arrested for allegedly going on a six-week-long killing spree that left four people dead on the city's South Side (search), police said Saturday.

Angela Ford Wright and Caroline Peoples are accused of killing three men in a series of "prostitution-related robberies," police spokeswoman JoAnn Taylor said.

A fourth victim, 30-year-old Ayesha Epps, was an acquaintance who was shot to death during an argument Thursday, Taylor said.

Each victim was shot in the back of the head, police said.

Wright and Peoples, both 26 and of Chicago (search), were arrested Friday morning. Each was charged with four counts of first-degree murder and four counts of armed robbery, Taylor said. Bond hearings had not been held as of Saturday.

The male victims were Jose Marquez, 38, who was killed at his home May 23; Kenneth Redick, 37, shot at his home June 2; and 24-year-old Kelvin Armstrong, who was killed in a parking lot June 18, according to police.