Mexico City - Was it an accident or was it an attack? Mexican authorities were beginning to investigate the cause of the plane crash that claimed the life of Mexican Interior Minister Juan Camilo Mourino and at least seven other people.

However, it was clear to observers that the death Tuesday night of the man who came second in Mexico's government hierarchy was the hardest blow that President Felipe Calderon has suffered since winning the presidency in late 2006.

Mourino, 37, born to a Spanish father and a Mexican mother, was a lot more than just the interior minister.

Indeed, before he took over that position in January he was already Calderon's closest friend in the circles of power. He coordinated the presidential campaign as well as Calderon's team in the transitional period that preceded his presidency.

Later, Mourino was the most powerful man behind the scenes as head of the president's office until Calderon put him in charge of the Interior Ministry, a key portfolio, from which he coordinated security issues, lobbied for reform of the energy sector and handled contacts with other parties.

Mourino, the father of three small children, was born August 1, 1971, in Madrid and became a Mexican citizen at 18. He was the son of Carlos Mourino, the president of the Spanish football club Celta Vigo.

His name was a favourite within the ruling National Action Party (PAN) to succeed Calderon in the president's office in 2012.

Mourino was a federal legislator and a mayoral candidate for PAN in the city of Campeche. As a legislator, he chaired the energy commission in the lower house of Congress when Calderon was in charge of coordinating the PAN bloc. Later, he was Calderon's adviser when the latter was energy minister.

Mexico's left-wing opposition, headed by former presidential candidate Andres Manuel Lopez Obrador, questioned alleged business operations by Mourino and his family in the energy sector, saying they took advantage of their position of power, as well as the legality of the minister's Mexican citizenship.

Mourino stressed he had given up his Spanish citizenship to become a Mexican as required by Mexican law for anyone holding public positions.

A sad Calderon bid Mourino farewell late Tuesday.

"[He was] one of my closest collaborators and one of my best and dearest friends," the president said.

Mourino's death reminded Mexicans of the death of Ramon Martin Huerta, public security minister under president Vicente Fox (2000-2006), who died in 2005 in a helicopter accident that was blamed on human error. (dpa)