Chihuahua state prosecutors' spokesman Arturo Sandoval said Edgar Lopez, 35, of El Paso, Texas, was killed Sunday along with two Mexican men when gunmen opened fire on a group standing outside a house.
On Saturday, a 26-year-old U.S. woman and an American boy were slain shortly after crossing an international bridge from El Paso. Giovanna Herrera and Luis Araiza, 15, were shot to death along with a Mexican man traveling with them just after 11 a.m., Sandoval said.
Sandoval said authorities also identified a 24-year-old woman killed Friday inside a tortilla shop as Lorena Izaguirre, a U.S. citizen and El Paso resident. A Mexican man was also found dead in the store.
Sandoval did not provide any information about possible motive in any of the slayings.
U.S. State Department spokesman P.J. Crowley confirmed three of the killings but did not have any information about Izaguirre. He said officials had been in touch with the victims' families but offered no other details.
Ciudad Juarez has become one of the world's deadliest cities amid a turf war between the Sinaloa and Juarez drug cartels. More than 2,000 people have been killed this year in the city, which is across the border from El Paso.
Also Monday, federal police said they arrested a U.S. man accused of being a member of the Aztecas gang, whose members work as hitmen for the Juarez cartel and operate on both sides of the border. Angel Martinez, 24, was arrested Saturday in Ciudad Juarez when he was traveling with another gang member, the department said.
Elsewhere, three city police officers were gunned down early Monday in a drive-by shooting as they patrolled the heart of Acapulco's upscale tourist district, authorities said.
Another officer was wounded, according to a statement from the Public Safety Department in southern Guerrero state, where Acapulco is located.
The officers were patrolling the Puerto Marques area around 1 a.m. when they were ambushed by suspects shooting assault rifles from inside a car, police said.
Violence continues to escalate in the Pacific resort city, days after Mayor Jose Luis Avila Sanchez warned people to stay indoors after dark. Ten other people were killed between Sunday and Monday around the area. Authorities also were trying to determine whether a burned corpse found in a car was the body of a Canadian businessman who disappeared last week.
Meanwhile, the remains of seven people were found Sunday in a mass grave in Nogales, on the Arizona border. Mayor Jose Angel Hernandez said a family walking near the site noticed what appeared to be part of a body sticking up in a riverbed. Officers recovered six bodies and a severed head in the grave. A seventh headless body was found nearby.
In the border city of Tijuana, state police seized more than 14 tons of marijuana in two vehicles at a house in the same neighborhood where gunmen killed 13 people at a drug rehab center 10 days ago.