Neither made it.
Gunmen chased down the two vehicles and opened fire in attacks that raised the chilling prospect that Mexico's cartels have dropped any reservations about killing American officials in their battle for the multibillion-dollar U.S. drug market.
Three adults with connections to the U.S. consulate were killed, and two children were wounded.
Mexico said U.S. intelligence, which is aligned with the murderous Juarez drug cartel. Authorities raised the possibility that only one of the families was targeted, while the other was chased because they both drove white SUVs. They offered no details of this theory.
Authorities in both countries said they don't know yet why the families were attacked.
"There is a concern at the possibility of attacks specifically aimed at diplomats stationed in the country, and that would be very serious," Ciudad Juarez Mayor Jose Reyes told a local radio station.
The FBI joined the investigation Monday, working with U.S. State Department agents and Mexican authorities.
Consulate spokesman Silvio Gonzalez said the party Saturday was thrown by the U.S. diplomatic mission in Ciudad Juarez. Were it not for that connection, the attacks would hardly have been remarkable in a city where 2,601 people were killed in drug violence last year.
After the slayings, the U.S. Embassy warned Americans to "delay unnecessary travel to parts of Durango, Coahuila and Chihuahua states."
Authorities would not say where the birthday party was held, but said it was not at the consulate. As the party wound down, the two families left separately.
Arthur H. Redelfs, an American who works as a jail guard in El Paso, was at the wheel of his white Toyota RAV4, driving along the broad riverside avenue leading to the Santa Fe Bridge across the border. His wife, Lesley A. Enriquez, a consulate employee who was four months pregnant, was at his side. Their baby girl was strapped into a car seat in back.
A Suburban fell in behind them, and Redelfs gunned the engine. They raced for a half-mile, coming within sight of downtown El Paso before Redelfs paused at the last intersection before the bridge. It was enough of an opening for the gunmen to slam into the driver's-side hood, then open fire.
Enriquez was killed by a single bullet in the head; her husband by two shots in the neck and arm. Their baby was unhurt, left wailing in the back seat.
Meanwhile, Jorge Alberto Salcido Ceniceros, a Mexican citizen who works at a factory south of the border, left the party with his two children, ages 4 and 7, in his white Honda Pilot. His wife, a Mexican citizen who works at the consulate, was not with them. They headed in the opposite direction, into Ciudad Juarez, where they lived.
Salcido speeded up when a car gave chase, racing down an avenue for 600 yards before the gunmen caught up with him. He was killed, and both children were wounded in a hail of bullets from an assault rifle.
"As to whether this was a particular incident directed at U.S. diplomats, I think we're not prepared to draw that conclusion yet," State Department spokesman P.J. Crowley said in Washington.
Enriquez's job involved aiding U.S. citizens, Crowley said. It was not immediately clear what kind of job Salcido's wife held at the consulate.
While several U.S. citizens have been killed in the drug war - most of them people with family ties to Mexico - it is rare for American government employees to be targeted.
The last high-profile slaying of a U.S. official in Mexico was the 1985 murder of Drug Enforcement Administration agent Enrique "Kiki" Camarena by Mexican drug traffickers. In 2008, attackers opened fire and threw a grenade at the U.S. consulate in the northern city of Monterrey, but the grenade didn't explode and nobody was hurt.
The violence in Ciudad Juarez had been creeping closer to the U.S. consulate before the latest attack, and on Friday the consulate declared a bar just down the block off-limits for security reasons.
The mayor speculated that the gunmen in Saturday's attack could have targeted one family and followed the other because it was in a similar car.
"One thing that could be significant is that the two SUVs were similar," he said. "The same type of vehicle, the same color, leaving the same party."
The mayor said Salcido may have worked previously as a police officer, but prosecutors said they could not confirm that. Police have been targeted in the drug violence that has made Ciudad Juarez one of the deadliest cities in the world.
The U.S. consulate in Ciudad Juarez, shut for a Mexican holiday on Monday, will remain closed Tuesday as "a way for the community to mourn the loss," Gonzalez said.
It was the second U.S. border consulate closed because of violence in the past month. The consulate in Reynosa, across from McAllen, Texas, was closed for several days in late February because of gun battles.
The State Department said it would offer housing allowances to employees at six U.S. consulates, including the one in Ciudad Juarez, who decide to send family members to safer areas. Crowley said about 100 dependents are eligible in Ciudad Juarez alone.
Crowley said that decision - based on the general level of violence rather than a specific threat - was made last week but wasn't announced until after the shootings.
Even in this country grown numb to drug violence, this was a bloody weekend, with nearly 50 drug slayings nationwide, reports CBS News correspondent Bill Whitaker. Nine people were killed in a gang shootout early Sunday in the Pacific resort city of Acapulco, where spring break was getting into gear.
Cartel watchers say the gangs, flush with guns and money, are sending a message.
"If you are in our way, either as a rival gang or a representative of authorities, this is your fate," Brian Jenkins, a senior advisor with the Rand Corporation, told Whitaker.
That message is heard loudly and clearly along the U.S.-Mexico border as rival cartels battle in the streets to control lucrative smuggling routes to funnel illegal drugs to the U.S. market, worth more than $40 billion a year. Even after President Felipe Calderon declared war on the cartels in 2006 and hit them with 45,000 troops, the trafficking and violence keeps growing, Whitaker reports.
Mexican marines and Navy personnel announced Monday they had launched a raid against an operations base run by the Zeta drug gang near Monterrey. A convoy of fleeing vehicles opened fire on a Marine helicopter following them, officials said. The Marines chased them down, some on foot, and killed eight suspects.
Elsewhere in Chihuahua state on Monday, running gun battles in the tourist town of Creel left seven people dead and two seriously wounded, while farther south prosecutors reported the bodies of five men were found on the side of a highway.
The bodies bore signs of torture and multiple gunshot wounds. They were found beneath an SUV that was apparently used to crush them.