A bomb exploded on a street in central Mexico City near the security ministry yesterday, killing one person and wounding two.
No group claimed responsibility for the blast and there was no warning.
The Mexican government is locked in a violent battle with drug gangs and has yet to catch left-wing rebels who planted small bombs at oil installations last year. Police were checking phone warnings that came in after the blast of other possible explosives left in nearby streets, Mexico City police chief Joel Ortega said.
Hundreds of officers in riot gear blocked roads around the bomb site and evacuated nearby buildings as police helicopters hovered overhead. Windows of buildings and parked cars were blown out and a large advertising awning was destroyed.
"We still have no message nor do we have the identity of these people," Mr Ortega said. "We are being cautious with the analysis of the materials we found. It is probably gunpowder."
Mr Ortega said the dead man had severe abdominal wounds and his hand was blown off, suggesting he may have been carrying the device when it went off. A woman was severely burned and a young man wounded in the blast near the ministry building, a couple of blocks from the capital's busy Reforma boulevard and near the bustling Zona Rosa district that is popular with tourists.
"We don't know if it was in one of their hands or whether they moved something," Ortega told Mexican radio.
Mr Ortega said it was unclear whether the device was left in a car or on the street. Another city police official on the ground said the blast could have been caused by a grenade.
President Felipe Calderon has deployed the army in a year-old battle with Mexico's powerful drug cartels.