Mexican police are trying to discover if a man killed in a shoot-out is one of the world's most violent drug traffickers.
Three men were left dead after a police chase two weeks ago, during Carnaval in Mazatlan.
Mexican and US officials are now trying to figure out if one of them was Ramon Arellano Felix, one of the FBI's 10 Most Wanted.
"We are investigating, looking for evidence," Mexican Attorney General Rafael Macedo de la Concha said.
He said his agency is "exchanging information" with US officials, including the US Drug Enforcement Administration.
"We have to exhaust all the tools that exist so that we can establish whether this hypothesis is a reality," he said.
Officials say the death of Arellano Felix would be a major blow to the Tijuana-based cartel allegedly run by Felix and his brothers.
They say Ramon Arellano Felix is the security chief for a group accused of killing about 300 police, prosecutors, judges, rivals and a Roman Catholic cardinal over the past 15 years while shipping tons of cocaine, marijuana, heroin and amphetamines across the border.