Monday's killing revealed some of the grittier aspects of one of the world's most violent cities, where about 6,000 drug-related murders were committed in 2009 and 2010.
Victim Maribel Hernandez worked as a distributor for the Diario de Juarez and PM newspapers. She had influence over which vendors got to sell the papers, and the La Linea gang - allied with the Juarez drug cartel - thought she was discriminating against paper vendors who paid protection money to their cartel, police said in a statement.
Local officials have said such street vendors sometimes also sell small quantities of drugs.
Federal police said that they had detained a suspect in the killing who told them he was paid 3,000 pesos ($250) to do the job.
Last week, federal police shot two girls who were traveling in a vehicle with someone who, moments earlier, had allegedly exchanged fire with police. One of the girls later died of her wounds. The Mexican National Human Rights Commission has launched an investigation.
Also Tuesday, federal police reported they had captured a leading enforcer for the Independent Cartel of Acapulco in the Pacific coast resort city of the same name. The enforcer allegedly took part in last year's kidnap-killing of 20 vacationers from the western state of Michoacan and in the killing and beheading of 14 men in Acapulco in early January.
Police identified the suspect as Miguel Gomez Vazquez, alias "The Cat," and said he was caught with an assault rifle and drugs.
The previously little-known cartel is believed to have broken off from the Beltran Leyva cartel due to infighting resulting from the arrest or death of many of that group's leading members.
Meanwhile, the Mexican navy said Tuesday that it had detained six suspected drug cartel gunmen on four separate occasions Sunday and Monday in the northern states of Tamaulipas and Nuevo Leon. One suspect was killed and two marines were wounded in one of the confrontations, the navy said in a statement.