MEXICO CITY, Mexico

Federal crime investigators began their probe Monday into what caused a massive oil pipeline explosion that killed 28 people and laid waste to parts of a central Mexican city.

Thirteen of the dead were children, and at least 52 people were injured. Scores of displaced residents were still in shelters, though officials have not given an exact figure.

Valentin Meneses, interior secretary for the state of Puebla, where San Martin Texmelucan is located, said 32 homes were completely destroyed and 83 partially damaged by the blast.

“A commission has been named that together with Pemex (Petroleos Mexicanos, the state oil company) is investigating what happened,” Meneses said.

At a news conference Monday night, Pemex director Juan Jose Suarez Coppel said that while preliminary signs still point to an attempted theft — including a ditch near the leak that appears to have been dug by humans — they cannot entirely rule out mechanical failure.

“There is a ditch that is not consistent with a leak from a mechanical failure, but until there is a metalurgical analysis, we cannot be sure it was not caused by a failure of the pipeline,” Suarez said.

A 55-foot (17-meter) section of the pipeline will be handed over to investigators for a study that could take three months, he said.

Francisco Fernandez Lagos, subdirector of refinery pipelines for Pemex, said the duct was inspected between 2005 and 2008 with no sign of any problem.

The scene of the disaster has been closed off and is being patrolled by police, Meneses added. Cleanup experts are also making sure there is no combustible material remaining in drains, which could threaten a new explosion.

Sunday’s blast in San Martin Texmelucan, about 55 miles (90 kilometers) east of Mexico City, left metal and pavement twisted and in some cases burned to ash in the intense heat.

Authorities say they believe thieves attempting to siphon oil caused a leak in the pipeline, sending black crude gushing into a street and flowing into a nearby river. It is not known what caused the oil to ignite.

Investigators found a hole in the pipeline and equipment for extracting crude, according to Laura Gurza, chief of the federal Civil Protection emergency response agency.

Authorities including President Felipe Calderon have promised to stop at nothing to bring whoever is responsible to justice. There have been no arrests so far, however.

Pemex has struggled with chronic theft, losing as much as 10 percent of all of its product. Criminals tap remote pipelines, sometimes building pipelines of their own, to siphon off hundreds of millions of dollars’ worth of oil each year, Pemex has said.

There have been 614 such thefts nationwide this year through November, compared with 400 last year, according to Pemex figures.

“The (overall) theft of fuel is less, but the number of illegal taps increases because we detect them faster,” Suarez said.

AP