For the second time in less than two weeks, heavily armed gunmen attacked a rehabilitation clinic for drug addicts in the volatile border city of Ciudad Juarez, authorities said today. At least 10 people, patients and therapists, were killed.

The gunmen escaped and authorities offered little in the way of motives in the Tuesday night shootings. Rehab clinics are often attacked as drug gangs pursue rivals or attempt to settle old scores.

It's the sixth drug treatment center attacked in Ciudad Juarez in the last 13 months. Eighteen people were slain in similar fashion on Sept. 2.

The attack Tuesday night on the Life Annex treatment center in a working-class neighborhood of Ciudad Juarez left dead the director, Dr. Iram Ortiz; a female patient he was counseling; another doctor; and seven male clients. At least two people were wounded, Enrique Torres, a spokesman for government security forces, said in a telephone interview.

A survivor, who watched the massacre from a hiding place inside the clinic, told reporters that as many as eight gunmen opened fire helter-skelter, shooting patients and anyone else in the building.

The shooting started around 10:15 p.m. Some neighbors said they at first confused the gunfire with fireworks exploding in the city's main square as part of Independence Day festivities that start late Tuesday and continue through today.

Despite a heavy military presence, Ciudad Juarez is Mexico's most violent city in a raging drug war that has claimed more than 13,000 lives since December 2006.

Scores of treatment centers for people suffering drug and alcohol abuse have sprung up in Mexican cities, reflecting this country's fast-growing addiction problem. Once just a pathway for drugs headed to the U.S., Mexico has become a consumer nation; the government says the number of addicts here has increased by 51% between 2002 and 2008.

But far from sanctuaries, some treatment centers are scenes of the violent world spawned by drug trafficking. Many of the clients come from the gangs fighting to control the drug market, and the battle in the streets carries over into the clinic. And often the centers are used as a fertile recruiting ground by traffickers.

Five other people were killed early today in Ciudad Juarez when gunmen attacked the Coco Bongo nightclub as customers were deep in celebration of Mexican Independence Day. Survivors told reporters that the gunmen appeared to single out their victims before escaping. It was the 23rd nightclub where patrons have been killed in the city this year, according to tallies by local newspapers.

In Tijuana, the bodies of six people were found stuffed in a car and badly burned.

wilkinson@latimes.com