The 22-year-old suspect has killed six men in the past four months because of his 'pact' with the popular pagan saint in exchange for "protection and prosperity".
The identity of the suspect, who was arrested last Saturdy, Aug 28, has not been revealed by the police.
In the Buenos Aires shantytown of Bajo Flores, the killer is known as "Marcelito".
"We're studying a total of about 10 cases. In some cases we have already obtained positive photographic recognition," another police official said.
"Saint Death" or "San La Muerte" is venerated in jails and among criminal gangs.
"Witness gave us the details of the pact (of the prisoner) with San La Muerte," a police spokesman told the media.