Investigators on Wednesday combed through the wreckage of a Russian passenger jet that slammed into a Ukrainian field during a severe thunderstorm, finding fragments of some of the 170 victims on board and the plane's two flight recorders.

Investigators hope the recorders can help explain what caused the Pulkovo Airlines Tu-154 to crash Tuesday — the third passenger airliner crash this year in the former Soviet Union.

Emergency officials said preliminary information suggested bad weather caused the crash about 30 miles north of the city of Donetsk in eastern Ukraine. The plane was flying to St. Petersburg from the Russian Black Sea resort of Anapa — a holiday destination popular with families.

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"Right now, it is difficult to determine the cause of the accident," Ukraine's Transport Minister Mykola Rudkovsky said in televised remarks.

The wreckage was found about an hour after the plane disappeared from radar screens in Sukha Balka, a village about 400 miles east of the capital, Kiev. On Wednesday, fragments of the plane — its engines, parts of the landing gear, the nose and chunks of the fuselage — were scattered around fields and a small forest. Authorities had stretched red tape around a 7,500-square-foot area.

Alexander Agayev, a Russian Emergency Situations Ministry official, said emergency workers had found fragments from 50 bodies in the wreckage by midday and were continuing with the grim search.

"The first task is to get the bodies out," Oleksandr Livochka, deputy prosecutor from the Donetsk region, said on Ukraine's NTN television.

He warned that identifying the bodies would be difficult, and DNA testing might be needed.

At least 50 relatives were expected to travel to the crash scene later Wednesday, said Vasily Nalyotenko, deputy general director of Pulkovo Airlines.

Of the 170 people on board, 45 were children, Pulkovo Airlines deputy director Anatoly Samoshin told reporters at the St. Petersburg airport. The list of passengers, many of whom were from St. Petersburg, appeared to include many families.

"Our citizens are very courageous, but there are very hard cases. In some cases the whole family has died," said Alexander Rzhanenkov, head of St. Petersburg's labor and social welfare committee. He said the city would pay about $3,700 to relatives and cover all burial expenses.

Preliminary information indicated there were five foreigners: two Germans and one each from the Netherlands, France and Finland, officials said.

Pulkovo Airlines said the 39-year-old captain was a very experienced pilot who had flown 11,900 hours.

Ukrainian officials said a storm with high winds, driving rain and lightning was raging through the region at the time of the crash. Russia's Emergency Situations Ministry spokeswoman Irina Andrianova, citing Ukrainian officials, said the plane was likely hit by lightning.

The pilot asked to make an emergency landing before disappearing from the radar screens at around 2:30 p.m., said Mykhaylo Korsakov, spokesman for the Donetsk Department of Emergency Situations Ministry. Rudkovsky also said that the pilot had asked for permission to change course by about 12 miles to the east, and was given permission.

Residents of Sukha Balka, a village north of Donetsk, found part of the plane's tail section and still-burning pieces of debris in a swampy field. Television footage showed scorched, smoldering land covered in small pieces of wreckage. Thick white smoke hung over the debris.

The crash was the third major incident involving Russia's aviation industry this year. It came less than two months after an Airbus A-310 of the Russian airline S7 skidded off a runway and burst into flames on July 9 in the Siberian city of Irkutsk, killing 124 people.

On May 3, an A-320 of the Armenian airline Armavia crashed into the Black Sea while trying to land in the Russian resort city of Sochi in rough weather, killing all 113 people aboard.

Russian-made Tu-154s are widely used by Russian airlines for many regional flights.