After denying the possible cause for the deadly violence, public safety officials said jailed leaders of the First Capital Command criminal group ordered the killings of more than 90 police officials so far this year as a reprisal for a crackdown on the drug trade, the British newspaper The Guardian reported Sunday.
So far in 2012, 92 former and current police officers have been killed.
Non-governmental organizations have said, however, mercenary groups of former and current police personnel accustomed to skimming profits from the drug trade share some of the blame.
The recent outbreak of violence -- including drive-by shootings and ambushes -- prompted schools to close early, changes in transit routes and demonstrations, The Guardian said.
"In view of the wave of violence in the city's south zone, the school's directors decided to send staff and students home early so as to assure their safety," Eliane Valerio de Souza, an administrative assistant at a vocational education school, told the newspaper Folha de S Paulo.
State and federal police said last week they would work together to form a new intelligence agency to counter the continuing threat posed by organized crime.
Sao Paulo's public security department reports 982 killings so far this year, up 10 percent from last year and higher than the total in Rio de Janeiro.
Gov. Geraldo Alckmin said the crime rate in the Sao Paulo metro area was lessening, but warned the problem wouldn't go away unless the national government took stiffer measures to control the influx of drugs and guns along Brazil's borders.