"He wanted to kill women -- white women, not black ones. He followed the victims, studying them closely before committing the crimes," said police commissioner Pedro Henrique Medina from the northern suburb of Nova Iguacu.
The man was identified as Sailson Jose das Gracas, a 26-year-old whom police describe as a psychopath.
They said they were checking his statements against past investigations, and so far they tally with the evidence.
"I observed them, I studied them. I waited for a month, sometimes a week, depending on the place. I tried to ascertain where they lived, what their families were like. I kept watch on their houses and then after a while went there at dawn, waited for my chance and entered," broadcaster Globo's G1 news portal quoted Gracas as saying.
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He explained he started out as a petty thief.
Gracas said he used a gun in contract killings but strangled his other victims.
He planned his crimes meticulously, wearing a rain jacket in order to conceal his identity and gloves in order not to leave fingerprints, adding he became addicted to murder.
"When I wasn't killing someone I got uptight. I would pace up and down at home. Killing calmed me down.
"I don't feel remorse -- if I go to jail for 10, 15, 20 years, then as soon as I get out I'll do the same thing all over again," he insisted.
Under Brazilian law, the maximum prison term is 30 years.
The country as never had capital punishment.