Cyntia Barrera and Manuel Rueda - MEXICO CITY (Reuters) - Mexican Interior Minister Francisco Blake was killed in a helicopter crash on Friday, a blow to the government as it fights powerful drug cartels.

Television images showed the scattered wreckage of the helicopter on a green hillside south of the capital where Blake and the seven others on board were killed.

President Felipe Calderon said conditions were cloudy when the helicopter came down so that an accident looked probable.

But in a televised address, he said no explanations for the crash could be ruled out as he paid tribute to Blake, calling him "a great Mexican".

"The investigations will be exhaustive and will consider all the possible hypotheses," Calderon said.

Mexico is locked in a brutal conflict against drug cartels that has killed 45,000 people in the last five years and Blake was a key member of Calderon's security team.

Locals near the crash site, a remote area only accessible by dirt roads, said the weather was poor when Blake's helicopter fell from the sky on Friday morning.

"We could hear the motor, which didn't sound good," said farmer Humberto Ramirez from the village of Caserio de Cortez, about 1.5 km (1 mile) away. "We couldn't see where it came down."

Blake is the second interior minister under Calderon to be killed in an air crash. The previous one three years earlier was declared an accident. Ironically, Blake lauded that very predecessor in the last tweet from his Twitter account.

Another top security official died in a helicopter crash in 2005, prompting one analyst to question security protocols for government aircraft and say the string of deaths could be down to more than just bad luck.

As interior minister, Blake was responsible for helping Calderon implement his strategy against drug gangs, as well as negotiating with opposition parties in Congress. Lawmakers held a minute's silence as a mark of respect for Blake.