CIUDAD JUAREZ, Mexico >> Mexico authorities say they are probing the deaths of four U.S. citizens who were shot to death in separate attacks in the border city of Ciudad Juarez over the weekend.

It was not clear Tuesday how many of the killings were related or what motivated the killers.

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.

The Ciudad Juarez killings represented the deadliest weekend for Americans in Mexico since Feb. 1, when four U.S. citizens were killed in different parts of the country. The largest previous single-city death toll for Americans was in June 2009, when three U.S. citizens were murdered in Tijuana.

State department figures show a steady rise in the number of Americans killed in Mexico. In the first six months of 2010, the latest figures available, 49 Americans were victims of homicide in Mexico, up from 37 for the same period in 2009 and 19 in the first half of 2008.

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.