In the case of the poisoned cocaine, the Argentine authorities came across a network of drug dealers, in which several police officers are said to be involved.
And investigators were able to identify the toxic substance in cocaine, which killed at least 24 people around two weeks ago.
Tjerk Bruhwiller
Correspondent for Latin America based in São Paulo.
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It's the substance carfentanyl, an opioid thousands of times stronger than morphine and used to calm elephants.
The poison was deliberately mixed with the cocaine.
A settlement between rival drug gangs is suspected to be behind the crime.
The masterminds are said to be a well-known gang led by imprisoned Max Alí Alegre, aka "Alicho".
A senior member of the gang was arrested shortly after the initial deaths.
The investigations are also directed against at least five police officers, including federal police officers and a lieutenant, who, as part of the gang, are said to have facilitated drug trafficking and collected protection money for it.
Several suspects were arrested.
Cooperation between drug gangs and police officers is nothing new.
During a raid last year, investigators came across money envelopes that should have been handed over to officials.
However, they were unable to secure any evidence as to who the recipients of the payments were.