Authorities had no immediate suspects or information on the victims. Ciudad Juarez, across the border from El Paso, Texas, is Mexico's most violent city, with at least 1,400 people killed this year alone.
Most of the homicides are tied to drug gang violence, which has taken a heavy toll across Mexico. Earlier the same day, gunmen ambushed and killed a senior security official in the home state of President Felipe Calderon.
Dozens of sobbing relatives rushed to the rehabilitation center to find out if their loved ones were among the dead. Soldiers and federal agents patrolled the streets surrounding the center in the Bellavista neighborhood.
Calderon sent thousands more troops and federal police to Ciudad Juarez earlier this year, but the surge has done little to stem the raging violence. The city is home to the Juarez drug cartel, which is battling other gangs for trafficking and dealing turf.
The government is struggling to revamp Ciudad Juarez's police force, which is plagued by corruption and the assassination of many of its officers. Other police have quit the force out of fear of being targeted.
The massacre capped a particularly bloody day in Mexico's relentless drug war.
Gunmen killed the No. 2 security official and three other people in Calderon's home state of Michoacan, where the government is locked in an intensifying battle with the ruthless La Familia cartel, blamed for a string of assassinations of police and soldiers.
Jose Manuel Revuelta, who was promoted less than two weeks ago to state deputy public safety director, is the highest-ranking government official killed in the wave of assassinations sweeping Michoacan, the cradle of La Familia drug cartel.
Attackers drove up alongside Revuelta as he headed home and opened fire, state Attorney General Jesus Montejano said.
Revuelta tried to speed away, but only made it a few blocks before he was intercepted by two vehicles. Six gunmen got out and sprayed Revuelta's car with bullets, killing him, two bodyguards and a truck driver caught in the crossfire, Montejano said.
An AP reporter at the scene saw the bodies of Revuelta and his bodyguards in the car, which had at least 15 bullet holes in the front windshield. Soldiers and federal police rushed to the site - just three blocks from the headquarters of the Michoacan Public Safety Department - and a helicopter circled overhead.
Soldiers and federal police have intensified their fight against La Familia since accusing the cartel of killing 18 federal agents and two soldiers last month. In the worst attack, 12 federal agents were slain and their tortured bodies piled along a roadside as a warning.
It was the boldest cartel attack yet on Mexico's government. Authorities said say La Familia was retaliating for the arrest of one of its top members.
The government has since rounded up more La Familia suspects, including Luis Ricardo Magana, who is alleged to have controlled methamphetamine shipments to the United States for the gang. Days before his capture, prosecutors detained the mother of reputed La Familia leader Servando "La Tuta" Gomez despite his threat to retaliate if police bothered his family. The woman was released after two days "for lack of evidence" of involvement in the cartel.
Calderon first launched his crackdown against drug cartels in Michoacan, sending thousands of federal police and soldiers to his home state after taking office in late 2006. Tens of thousands more have since been deployed to drug hotspots across Mexico.
Drug gang violence has since surged, claiming more than 13,500 lives, including more than 1,000 police officers.
Calderon defended his battle against drug trafficking in a speech to Congress on Wednesday. He said the government has taken on the cartels as no previous Mexican administration has dared to do.
"As never before, we have weakened the logistical and financial structure of crime," the president told legislators.
The federal Attorney General's Office, meanwhile, announced the arrest of its two top officials in Quintana Roo, a state on the Yucatan Peninsula, for allegedly protecting the Gulf and the Beltran Levya drug cartels.
Officials provided no further details on the allegations against the prosecutors, who were ordered jailed by a court Wednesday pending the investigation.
Associated Press Writers Gustavo Ruiz in Morelia, Michoacan, Manuel de la Cruz in Tuxtla Gutierrez, Chiapas, and Alexandra Olson in Mexico City contributed to this report.