MEXICO CITY (Reuters) - Mexico will search another three days beneath the rubble for possible survivors of the September 19 earthquake even though it is unlikely rescuers will find anyone alive, the country's chief of emergency services said on Monday.

Luis Felipe Puente, coordinator of Mexico's Civil Protection department, also told Reuters that the government has instructed prosecutors to investigate newly constructed buildings that collapsed in the quake for code violations, including a school where 19 children and seven adults died.

"I can say that at this time it would be unlikely to find someone alive," Puente said in an interview, referring to 43 missing people being sought at four disaster sites in Mexico City.

The 7.1 magnitude quake struck one week ago, killing 326 people including 187 people in Mexico City, damaging 11,000 homes, of which about 1,500 will need to be demolished, Puente said.

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About 10 percent of damaged buildings were constructed after strict building codes were enacted in the wake of the devastating 1985 earthquake that killed an estimated 10,000 people, Puente said, leading officials on Monday to instruct prosecutors to open investigations.

"The Mexico City mayor and the national government have already ordered judicial investigations to determine who was responsible for new construction that did not meet the requirements," Puente said from Civil Protection headquarters, where a roomful of technicians monitored seismic activity and tropical storms on an array of screens.

A school that collapsed in southern Mexico City, killing 19 children and seven adults, will be among the subjects of the investigations, Puente said.

Officials who approved the school building, the construction company and the owner of the property all could be held accountable if any violations are discovered, Puente said.

The largest search and rescue effort was under way at an office building in the Roma section of Mexico City where 40 people may be buried, based on families who have reported their relatives missing, Puente said.

A person was believed missing at each of three other buildings in the capital where search operations are in progress, Puente said.

Dogs trained to pick up the scent of survivors have yet to find any signs of life at the search sites, he said.

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Asked how mucher longer search and rescue operations would continue, Punete said, "As of today, we have agreed to another 72 hours."

(Editing by Simon Cameron-Moore)