A bomb exploded Friday on a street in central Mexico City near the security ministry, killing one person and injuring 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. Last year left-wing rebels planted small bombs at oil installations.
Officers are checking warnings that were phoned in after the blast about other possible explosives left in nearby streets, Mexico City Police Chief Joel Ortega said.
Hundreds of officers in riot gear blocked roads around the bombing site and evacuated nearby buildings as police helicopters hovered overhead. Windows of buildings and parked cars were shattered, and a large advertising awning was destroyed.
"We still have no message nor do we have the identity of these people," Ortega said of the perpetrators. "We are being cautious with the analysis of the materials we found. It is probably gunpowder."
Ortega said the dead man had suffered severe abdominal wounds and his hand was blown off, suggesting he may have been carrying the explosive device when it went off.
A woman was severely burned and a young man hurt in the blast near the Ministry of Public Security, a couple of blocks from the capital's busy Reforma boulevard and near the bustling Zona Rosa district popular with tourists.
"I was working here one block down when I heard a really strong blast that shook everything," said Alfredo, a young man cleaning car windshields at a street corner.
Bomb squads were checking cars in the street.
President Felipe Calderon has deployed the army in an effort to crack down on Mexico's powerful drug cartels. Police have made several arrests in the capital and seized suspected gang members found with large arsenals in the year-old battle.
Mexico is not home to any major terrorist groups, but it was hit last year by a series of bomb attacks on oil installations by a small left-wing guerrilla group. No one was killed in those blasts.
The Marxist-inspired Popular Revolutionary Army, or EPR, had lain dormant for several years until last year, when it badly hurt Mexican industry with the attacks on natural gas and fuel pipelines.