TEHRAN, Iran – Investigators have recovered the three black boxes belonging to a Russian-made jetliner that crashed in northwest Iran shortly after taking off from the capital, killing all 168 on board, authorities said Thursday.

Remains of the dead — many badly burned and damaged — were brought to Tehran for identification, as relatives from Armenia, the passenger jet's destination, headed for Iran to retrieve their loved ones' bodies.

Khristofor Sogomonian, whose father was among those killed, said he would "try to find anything and to commit his body to the soil."

Diana Sarkisian told The Associated Press at the airport in Armenia's capital Yerevan, that her cousin was aboard the plane as the first leg of a trip to her husband in the United States. "I had hoped that she was alive, but now all doubts have fallen away," said Sarkisian, an Iranian national.

The black boxes — containing the plane's cockpit voice and flight data recorders — will be key to determining the cause of the crash, which remains unknown. Witnesses said the plane's tail, where the engines are located, was on fire before it went down nose first, plowing a long trench into agricultural fields outside the village of Jannat Abad, and the plane was blasted to bits. Parts of the trench were up to four meters (yards) deep.

Chief investigator Ahmad Majidi said one of the recovered boxes was damaged, state radio said. The boxes would likely be sent to the aircraft's Russian manufacturers for analysis, he said.

A team of Russian air accident experts was due to arrive in Iran on Friday to help in the investigation of the latest crash, civil aviation spokesman Reza Jafarzadeh told the semi-official news agency ISNA.

Majidi initially said two black boxes were found and that investigators were searching for a third. Later state TV reported that the third was found, some 4 kilometers (2.5 miles) away from the crash. Black boxes, built to survive crashes and intense fires, record a plane's performance, like speed and altitude, as well as communications between the cockpit crew or with air traffic controllers.

The crash of the Tupolev jet leased by Caspian Airlines was the latest in a string of air disasters in recent years that have highlighted Iran's difficulties in maintaining its aging fleet of planes. Iranian airlines, including state-run ones, are chronically strapped for cash, and maintenance has suffered, experts say.

U.S. sanctions prevent Iran from updating its 30-year-old American aircraft and make it difficult to get European spare parts or planes. The country has come to rely on Russian aircraft, many of them Soviet-era planes that are harder to get parts for since the Soviet Union's fall.

Iranian civil aviation agency chief Ali Ilkhani said the Tu-154M that crashed was built in 1987, was bought by Iran in 1998 and had an overhaul certificate valid until 2010, state TV said.

Most of the passengers were Iranians, including 42 from Iran's large ethnic Armenian community, as well as 11 members of Iran's national youth judo team.

Five Armenian citizens were among the dead, Armenia's Foreign Ministry said in a statement, along with two Georgians, including a staffer from the Caucasus nation's embassy in Yerevan.

Armenia on Thursday announced a one-day national state of mourning to mark the death of its citizens in the crash. Flags were flying at half-mast on government buildings and Armenian embassies abroad. Local radio and TV have canceled entertainment programs in a show of respect.

Iran's national airline sent a Boeing 747 to Yerevan Thursday to help take victims' relatives to Tehran. Caspian Airlines' representative in Armenia, Arlen Davudian, said victims' relatives would be provided hotel rooms and transportation to the crash site.

Victims' relatives, he added, would be paid compensation of at least euro32,000 ($45,216).

At the site of the crash on Wednesday, flaming wreckage, body parts and personal items were strewn over a 200-yard (meter) area. Firefighters put out blazes from the crash, but smoke smoldered from the pit for hours after as emergency workers searched for the data recorders and other clues to the cause.

The Tu-154M jet had taken off from Tehran's Imam Khomeini International Airport. It crashed at 11:30 am, about 16 minutes after takeoff, outside Jannat Abad, near the city of Qazvin, around 75 miles northwest of Tehran.

The crash is Iran's worst since February 2003, when a Russian-made Ilyushin 76 carrying members of the elite Revolutionary Guards crashed in the mountains of southeastern Iran, killing 302 people aboard. That crash was a sign of how maintenance problems have also affected Iran's military.

Caspian Airlines is an Iranian-Russian private joint venture founded in 1993.