Mexico City (CNN) -- Authorities found two severed heads along a highway in Mexico City on Monday morning, the city's attorney general said.
The heads were inside a car near the border dividing the city from neighboring Mexico state, Attorney General Miguel Mancera told reporters. The nation's defense ministry was also nearby.
A lengthy message was left beside the heads, Mancera said, detailing that a drug gang known as "The Hand With Eyes" was responsible.
"It ends saying that the territory of 'The Hand With Eyes' is so extensive that it has reached the metropolitan area and hasn't disappeared," Mancera said.
Police in Mexico City arrested the gang's suspected leader, Oscar Osvaldo Garcia, also known as "The Hand With Eyes," in August.
Mancera said Monday that authorities remained committed to tracking down members of the gang and stopping its growth.
Authorities were still working Monday to identify the victims.
Beheadings have become a signature sign of the brutally violent turf battles between drug cartels that have plagued cities across Mexico.
Last week authorities in the beach resort city of Acapulco made a similar discovery, finding five severed human heads inside a small wooden crate, Guerrero state authorities said.
But such violence is rare in the country's capital.
"Mexico City has been surprisingly calm in the midst of the turbulence of this conflict with organized crime, while other major cities are experiencing major upturns in violence," said Eric Olson, a security expert at the Mexico Institute of the Woodrow Wilson International Center for Scholars in Washington. "It hasn't exploded the way it has (elsewhere). There's always that fear of that happening, because there's nothing to guarantee that it won't."
CNN's Krupskaia Alis, Rene Hernandez, Rafael Romo and Catherine E. Shoichet, and CNNMexico.com contributed to this report.