Mexican gas and oil pipelines were attacked in six places before dawn Monday, causing explosions, fires and gas leaks that forced the evacuation of thousands of people.

The blasts reverberated for miles. No direct injuries were reported, although civil defense agencies said two women in their 70s who lived nearby died of heart attacks shortly afterward.

A small, shadowy leftist group linked to similar attacks in July left a note claiming responsibility, a police official in the Gulf coast state of Veracruz told The Associated Press. He spoke on condition of anonymity because he was not allowed to be quoted by name.

The note was found alongside at least one undetonated explosive device uncovered by soldiers in a swampy area about 550 yards away from a highway toll booth 25 miles north of the port of Veracruz, the official said.

The government did not immediately verify the information. Interior Secretary Francisco Ramirez said the federal Attorney General's Office was trying to determine who was responsible for the "premeditated acts."

"Pemex's fundamental installations are adequately protected by our armed forces, and we will do our utmost to find those responsible," Interior Secretary Ramirez said.

The six blasts happened about 2 a.m., according to a statement from the Mexican state oil monopoly Petroleos Mexicanos, or Pemex. The company immediately shut down the affected lines as well as an extra line in the area as a precaution.

Flames from the fires could be seen up to six miles away, said Pedro Jimenez, who was packing his family into a truck to leave. "You could see the fields of crops lit up."

Dozens of families lined roadways to evacuate to local shelters.

Pemex said domestic gas and gasoline service would not be affected.

At four sections of the pipelines, fires broke out, while at others leaking gas prompted fears of explosions and forced civil protection authorities to evacuate several communities including Ciudad Cardel and Antigua, said state Civil Protection Deputy Director Ranulfo Marquez.

The explosions also prompted authorities to close two main highways.

"We still have a gas leak in the area of Ciudad Cardel," Marquez said. "There is still a risk."

Authorities also were checking to see if any gas had leaked into the Chiquito River, near the city of Nogales, said Nogales Mayor Marcelo Aguilar.

Starting Sunday evening, residents reported smelling gas from the pipelines.

The explosions could be felt up to 12 miles away, Marquez said.

The July attacks forced at least a dozen major companies, including Honda Motor Co., Kellogg Co. and The Hershey Co., to suspend or scale back operations.

Those attacks sent the Mexican government scrambling to increase security at "strategic installations" across Mexico. It was not clear what security measures were in place Monday.

Mexico is a major oil producer and exporter, with oil and related taxes accounting for over a third of the federal government's revenue. The United States imported 12.7 million cubic feet of natural gas from Mexico in 2006, about 0.3 percent of total imports that year.

Natural gas futures rose 5.5 cents to $5.56 per 1,000 cubic feet in late morning trading on the New York Mercantile Exchange after initially rising more than 20 cents on news of the explosions. Mexico's stocks opened lower amid the reports of sabotage.