U.S. authorities arrived in Ciudad Juarez, across the border from El Paso, Texas, on Monday night to "collaborate and inquire about any advances in the cases," prosecutors' spokesman Arturo Sandoval said.
Sandoval said he couldn't reveal which U.S. authorities are involved because of the ongoing investigation. But he said two of the victims, Giovanna Herrera, 26, and Luis Araiza, 15, had criminal records. He would not specify what offenses.
The two were shot Saturday along with a Mexican man traveling with them just after they crossed an international bridge from El Paso, 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. 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 motives in any of the slayings.
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.
The attacks 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, up from 37 for the same period in 2009 and 19 in the first half of 2008.
The vast majority were in border cities such as Ciudad Juarez, Tijuana and Mexicali, which have also been the hardest hit by drug violence. In some of the killings, the Americans apparently were in the company of Mexican friends, relatives or acquaintances who were the targets.
Other Americans have been killed by stray bullets, and in at least one case, Americans were directly targeted by a drug gang: U.S. consular employee Lesley A. Enriquez and her husband, Arthur H. Redelfs, who were gunned down in their white SUV on a Ciudad Juarez street March 13.
Suspects later told investigators that the Azteca gang ordered the killings, claiming Enriquez helped rival gang members get visas. Investigators deny that Enriquez was involved with drug gangs.