Authorities in Mexico say a letter bomb sent to a professor at a technical university slightly wounded another teacher when it was opened.
Pachuca Polytechnic University says a teacher who belongs to the school's security committee was slightly burned on his hands when he opened the envelope. The letter had been identified as suspicious by the professor it was addressed to.
Officials have not said whether Thursday's explosion was connected to two similar bombs sent this year to other technology researchers.
A radical group that opposes nanotechnology claimed responsibility for those bombings. In a manifesto, the group praised the "Unabomber," whose mail bombs killed three people and injured 23 in the United States.