(CNN) -- Chilling video captured by a security camera shows more than a dozen armed men carrying out a shooting spree in broad daylight that left eight people dead last month in Mexico's Chihuahua state.

In the video obtained by CNN on Saturday, the men -- believed to be hit men for a drug cartel -- are seen opening fire with assault rifles into nearby vehicles and a housing complex.

A 14-year-old girl was among the eight victims of the March 15 killings in the town of Creel, according to Carlos Gonzalez, a spokesman for the state attorney general's office.

"We don't know what group these hit men are part of, but here in Chihuahua there are two warring groups: the Sinaloa Cartel and Juarez Cartel," Gonzalez said. "We're working on trying to figure out which group was responsible."

Gonzalez said there is no evidence to suggest that the victims were involved in drug activity, adding that the motive for the shootings is unclear.

He said the video -- captured by a state public security camera -- is "important" evidence in the investigation.

"This is evidence to identify these people and it's part of the ongoing investigation to find them," said Gonzalez, who is the lead investigator in the case.

In the video, gunmen are shown approaching the passenger side of a vehicle to receive what appears to be an order for the killings. A man holding a large gun in the passenger seat also is seen taking a white substance and putting it in his nose several times.

Another portion of the video shows two gunmen intercepting a car and sequestering the driver. Later in the video, the men shoot two people in an SUV.

In some of the most shocking footage, armed men are seen running across a field before reaching the doors of a housing complex and opening fire into the doors and windows of the residence.

The shootings happened over a period of an hour early on March 15, Gonzalez said.

"There are very few authorities in this area to police this kind of activity," Gonzalez said.

The images of the gunmen operating freely in the mountainous northwest part of Chihuahua highlight just how lawless rural parts of Mexico have become as the country works to combat an increasingly violent drug war.

In January, in a case of mistaken identity, 15 people, mostly teenagers, were killed when gunmen attacked a house party in Ciudad Juarez, Mexico.

And late last month, 10 children, youths and young adults between the ages of 8 and 21 were gunned down, presumably by drug traffickers, in the northern Mexican state of Durango.