WARSAW Poland opened an investigation on Thursday into its worst military plane disaster, in which 20 air force personnel were killed.
The Spanish-made CASA C-295 transport plane crashed in a forest on Wednesday evening on its approach to the Miroslawiec military airport in northern Poland.
Four crew died along with 16 passengers, including an air force general and the commander of the base, who were returning from an air safety conference.
The head of Poland's air force, General Andrzej Blasik, said the special commission would require several days to produce an initial report.
Despite earlier media reports, officials said they had not yet recovered the flight data and cockpit voice recorders.
President Lech Kaczynski was due to announce a national period of mourning after cutting short a state visit to Croatia, his office said.
One of the firemen called to the scene told a local television station that the twisted wreckage hardly resembled an airplane.
The aircraft that crashed had entered service in August. It was one of 10 owned by the Polish air force, which also uses them to transport personnel and equipment to Iraq and Afghanistan.
The rest of the fleet were grounded pending the investigation.
The C-295 is a twin-turboprop medium-sized air transporter made in Spain by EADS CASA, a unit of the European aerospace group EADS.
(Editing by Kevin Liffey)