Ueberlingen, Germany ? A Russian charter jet filled with youngsters headed for a vacation to Spain slammed into a cargo plane, killing more than 70 people and scattering flaming wreckage on the shores of a picturesque lake.

All aboard were believed dead after the collision at about 36,000 feet. “At such an altitude, it would be a wonder if anyone survived,” said Wolfgang Wenzel, a police spokesman for the state of Baden-Wuerttemberg.

The crash prompted an angry clash between Russian aviation authorities and Swiss air traffic controllers. The controllers said once the plane entered their air space they told it to descend, but that when it did, it was too late.

Sergei Rudakov, the head of Domodedovo airport, denied the pilot of the Bashkirian Airlines Tu-154 had caused the collision late Monday night and suggested it was perhaps the other plane’s pilot. He spoke on a report broadcast on Russia’s RTR television.

Rescue crews in helicopters using infrared cameras worked through the night in search of bodies or survivors, and hundreds of people searched the low rolling hills around Lake Constance. The lake, shared by Austria, Germany and Switzerland, is one of Europe’s biggest, and is a popular vacation spot dotted with sailboats in the summer.

More than 800 rescue workers were spread across the 20-mile-wide crash site Tuesday. By daybreak, 12 bodies had been recovered, and investigators had located the flight data recorder from the Tu-154, as well as smoldering pieces of both aircraft. Even though none of the parts were believed to have fallen into Lake Constance, 22 boats patrolled the waters looking for wreckage or telltale jet fuel slicks.

The region is on Germany’s border with Switzerland and Austria, about 135 miles south of Frankfurt.

The Tu-154 was carrying a group of children and teen-agers from Bashkorstotan, a Russian republic in the southern Ural Mountains.

The children of high-ranking regional officials from Bashkorstotan, they were part of a group traveling to the Costa Dorada outside Barcelona for a summer holiday on a trip sponsored by UNESCO, Russia’s Interfax news agency quoted Bashkortostan Prime Minister Rafael Baidavletov as saying. The Paris-based United Nations Educational, Scientific and Cultural Organization said they could not immediately confirm their agency’s involvement.

At the airport in Ufa, the capital of Bashkortostan, parents of the children were trying to get to Germany.

“If only they had flown on time, nothing would have happened,” said a woman who was identified as the mother of 11-year-old Bulat Biglov. The group was supposed to have left Saturday for Spain, but missed their connection and had to wait. The woman, shown on NTV television, was in tears.

Tuesday, Wednesday and Thursday were set aside for mourning in Bashkortostan.

Russian President Vladimir Putin sent his condolences to the relatives of the victims, and dispatched investigators and the Russian general consul in the Germany city of Bonn to the crash scene.

Russian officials at the emergency situations ministry and the tour agency that helped organize the trip said eight of the children were younger than 12. Forty-four of the children were between 12 and 16 years old.

The Boeing 757 was from the DHL package delivery service, which has headquarters in Brussels, Belgium, and San Francisco. It was en route from Bahrain to Brussels, according to the company’s Web site.

Swiss air traffic controllers repeatedly asked the Tu-154 pilot to lower his altitude, but he did not respond, said Ulrich Mueller, the Baden-Wuerttemberg state transport minister. The DHL plane changed course, but it was too late to avoid the crash late Monday night, Mueller said.

Russian aviation authorities angrily denied pilot error as a possible cause, saying the Tu-154 pilot had years of experience and spoke English well so he would have understood instructions to descend.

Anton Maag, chief of the air traffic control tower in Zurich, said the air traffic controller _ described as a very experienced member of staff _ gave the first descent order to the Russian plane about two minutes before the collision. “This is normally sufficient,” he said.

But the Russian plane only began to decrease altitude after a third request, Maag said. At the same time, the DHL plane’s automatic collision warning system inexplicably issued an order to descend, and pilots are obliged to immediately follow these instructions, Maag said.

At Moscow’s Domodedovo airport, Bashkirian representative Sergei Rybanov said 52 children, five adults and 12 crew were aboard the Russian plane. All flew into Moscow’s Sheretmeyevo airport on Saturday, but when they missed their connection requested that the airline organize a special flight to Barcelona.

Tatiana Ostapenko said her Moscow travel agency helped organize a group of 49 people for the flight _ 44 children and five adults accompanying them. One adult worked for her Soglasiye Tourist company, she said. “We are in shock.”

Axel Gietz, head of DHL corporate affairs in Brussels said both people aboard the cargo jet, the British pilot, Paul Phillips, and his Canadian co-pilot, Brant Campioni, were killed.

Another DHL spokesman, speaking on condition of anonymity, said their aircraft was built in 1990 and purchased by his company in 1999 from British Airways, which had used it as a passenger jet. It was equipped with a traffic collision avoidance system and had been subject to regular inspections and maintenance like all the company’s planes, he said.

“There were no indications of any technical or operational problems with the aircraft,” he said.

Authorities said nobody on the ground was harmed, even though large chunks of the plane fell close to homes.

Witnesses said they heard a noise like thunder and saw a fireball erupt in the night sky, then saw large and small pieces of wreckage falling to the ground and into Lake Constance. Scattered fires were sparked in the rural area, but there were no casualties on the ground, authorities said.

Dirk Diestel, 47, was changing his child’s diaper shortly before midnight when he looked up through a skylight and saw a huge fireball in the sky.

“Immediately I thought that something horrible had happened,” he said. When he went outside, a large piece of one of the plane’s landing gear was lying a few feet from his home.

Bashkirian Airlines has eight Tu-154s in its fleet of 39 Soviet-designed planes. It mainly serves Russia and former Soviet republics, with some charter flights to other destinations.

The three-engine Tu-154, first put into commercial service in 1972, is the workhorse of Russia’s domestic airlines and widely used throughout the former Soviet Union and Eastern Europe, as well as in China.

A Tu-154 crashed in the Siberian city of Irkutsk last July, killing all 143 aboard. Another crashed on takeoff from Irkutsk in 1994, killing 124 people. The plane reportedly was overloaded.

A Tu-154 belonging to China Southwest Airlines crashed in China in 1999, killing all 61 people aboard. A German-owned Tu-154 collided with a U.S. Air Force C-141 off the coast of Namibia in 1998, killing 33 people, and in 1997 a Tajik Tu-154 crashed en route to the United Arab Emirates, killing 85.