The bodies of three women and an 11-year-old girl were found in an apartment in the Argentine city of La Plata, 60 kilometers (37 miles) south of Buenos Aires, police said Sunday.
They had all been murdered.
The bodies were found after a neighbor called emergency services when blood was seen seeping out from under the apartment door, which was not completely closed.
Police said that, according to initial reports, the women had all been stabbed to death and the apartment was in complete disorder.
The victims included a woman in her 60s, her 30-year-old daughter, her 11-year-old granddaughter and another woman, age 30, authorities said.
Several hours after the bodies were found, prosecutor Alvaro Garganta announced that a 27-year-old man identified as Oscar Martinez had been arrested as a suspect in the crime.
According to press reports, the arrested man was the boyfriend of one of the victims.