By Mica Rosenberg

MEXICO CITY, Feb 18 (Reuters) - Mexican police tightened security in the capital on Monday in the wake of a fatal bomb attack, but could still not say for sure whether drug gangs were behind the explosion.

Mexico City police chief Joel Ortega said more police helicopters would fly over the capital, one of the world's largest cities, and bomb squads would patrol busy areas after a bomb killed one person on Friday and wounded two others.

"We want to have rapid response capabilities at strategic points," Ortega told a news conference. Police checked cars leaving the city over the weekend.

The small homemade bomb, activated by a cell phone, went off prematurely on a street near Mexico City's security ministry, killing a man thought to be handling it and seriously burning a woman with him.

The city's top prosecutor, Rodolfo Felix, said on Friday the blast could have been a bungled attack by Mexico's powerful drug cartels and appeared to rule out a small left-wing guerrilla group that bombed fuel pipelines last year.

But Ortega said on Monday it was still too early to say for sure who was behind the bombing.

The man killed in the blast lost his hand and had severe abdominal wounds, suggesting he had been holding the device at the time. The injured woman asked questions about her companion as she was taken to a hospital.

Mexico's feuding drug cartels killed more than 2,500 people last year in brutal gangland-style executions, as President Felipe Calderon deployed the army to crush them. However, drug gangs have not been known to use bombs so far.

Since Friday's attack, authorities in the capital of 18 million people have received nine calls warning of other bombs, but all turned out to be false alarms, police said. (Editing by Catherine Bremer and Todd Eastham)

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