Thick fog likely caused the helicopter crash on Friday that killed Mexican Interior Minister Francisco Blake and all seven others on board, the country's transportation minister said on Saturday.

An exhaustive investigation is under way, but the initial results indicate the crash was an accident, Communications and Transport Minister Dionisio Perez-Jacome told a news conference.

At this time we have no indications, no evidence ... that would make one suspect that this is anything other than an accident, he said.

That contradicted an earlier report in a Mexican newspaper, citing a preliminary investigation, that pilot error could have caused the crash.

As interior minister, Blake was responsible for helping Mexican President Felipe Calderon in the country's fight against powerful drug cartels.

Blake, 45, was the second interior minister under Calderon to be killed in an air crash, which fuelled some Mexicans to speculate on Twitter about the causes of his death.

Investigators confirmed the pilot was flying manually, using his own vision rather than instruments to guide the craft, but they said that was standard given the good weather conditions when the helicopter left Mexico City.

The helicopter, which was transporting Blake to an event in Cuernavaca about 60 miles (100 km) south of Mexico City, is believed to have encountered low-lying clouds outside the capital, Perez-Jacome said.

At that point, to seek better visibility the pilot diverted from the planned route, but floundered in dense fog southeast of the capital and crashed on a remote hillside.

Initial checks of the damaged helicopter showed no damage from an explosion or fire, Perez-Jacome said.

Further investigations will seek to confirm the presence of fog in the area where the helicopter crashed, investigators said.

Mexico has requested help from the U.S. National Transportation Safety Board as well as the French office in charge of investigating civil aviation accidents, Perez-Jacome told reporters late on Friday.

This investigation will be exhaustive and will consider all the available evidence, Calderon said at a memorial service for the crash victims on Saturday.

Calderon called Blake an admirable public servant and a very close friend.

The president called off his planned trip to Hawaii for a summit of Asia-Pacific nations after the crash.

(Reporting by Elinor Comlay and Luis Rojas Mena; Editing by Peter Cooney.)