The sons of imprisoned drug lord Joaquin "El Chapo" Guzman are suspected of launching an ambush on a military convoy in Mexico that left five soldiers dead, officials say.
The assault in the northern state of Sinaloa involved grenades and assault rifles.
The attackers succeeded in freeing a wounded drugs suspect who had been captured earlier and was being taken to hospital, officials say.
At least 10 people were injured.
Sinaloa is the stronghold of the drugs gang formerly headed by Joaquin Guzman. Nicknamed "El Chapo", he was recaptured in January.
Officials believe the ambush may have been organised by his sons, who are believed to have taken over the running of the cartel.
"Up this point we are not certain about this group, but it is very probable that it was the sons of Chapo," said local military commander Gen Alfonso Duarte.
BBC Mexico correspondent Juan Paullier said that the soldiers were travelling in the early hours of Friday in two vehicles escorting a Red Cross ambulance when they were attacked.
Up to 60 gunmen set upon the vehicles, beating the soldiers and freeing a drugs suspect who was being taken to hospital.
Officials say the suspect freed by the gunman has been identified as Julio Oscar Ortiz Vega, also known as "El Kevin".
The gunmen stole the ambulance and a military truck burst into flames during the assault.
Mexican authorities believe the sons of Guzman have been involved in other violent episodes in recent weeks.
In August, one of them was kidnapped by a rival group. Another son, Ivan, is also said to be involved in cartel activities.
Officials say Guzman's brother, Aureliano Guzman Loera, also known as "El Guano", is struggling for control of drug production against the Beltran Leyva drug cartel in the remote area.
Guzman was arrested in January after six months on the run following his escape through a tunnel in his jail cell.
He had already escaped a maximum security facility once before, spending 13 years at large.
A Mexican judge is considering the case to have Guzman extradited to the US.
But Guzman is expected to appeal against any decision to extradite him and his lawyers say the process could take years to come to a conclusion.