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.
Mexico's peso and its IPC stock index lost ground after Blake's death, although the peso firmed again after Calderon said an accident looked likely.
Calderon's presidency has been dominated by his decision to send in the army against drug gangs shortly after he took office in December 2006.
He canceled his appointments for the day and called off a planned trip to Hawaii where he was due to take part in an Asia-Pacific Economic Cooperation (APEC) summit.
US President Barack Obama, himself on the way to Honolulu for APEC, called Calderon from Air Force One to express his shock and sorrow at Blake's death.
The soaring death toll has battered support for the ruling National Action Party (PAN) ahead of a presidential election next July, and this year dragged down Calderon's approval ratings to their lowest levels of his term.
Born in Tijuana on Mexico's border with the United States, Blake trained as a lawyer and served as a deputy for his home state of Baja California. He became interior minister in 2010.
"There is great sadness, firstly for the loss of a great friend, but also because this is a very severe blow in the midst of a war against criminals inside Mexico," said Socrates Bastida, a leader of the PAN in Baja California.
On November 4 2008, then interior minister Juan Camilo Mourino and several others died when their small plane crashed near a major Mexico City boulevard. Investigators concluded the plane was flying too close to a much bigger jet plane ahead of it.
In his last tweet on November 4, Blake said: "Today we remember Juan Camilo Mourino three years after his passing, a human being who worked toward the creation of a better Mexico."
In 2005, Ramon Martin Huerta, public security minister in the previous PAN administration, was killed in a helicopter crash just outside the city. It was declared an accident too.
"Statistically, it is starting to become very difficult to blame these accidents on human error," said Armand Peschard-Sverdrup, a Mexico expert at the Washington-based Center for Strategic and International Studies.
"The Mexican government has to review its security protocols for all aircraft and vehicles ... and you have to be open to the idea that this was not a random accident."
The government's website lists a fleet of eight planes and nine helicopters, which are used frequently by officials.