New York ? Researchers believe the second hijacked plane to hit the World Trade Center tower was traveling about 100 mph faster than the first, according to a published report.

Investigators are focusing on the speed of the two planes as they seek to explain what caused the south tower to collapse first, even though it was hit later, The New York Times reported Saturday.

Researchers have closely studied videos, sound recordings and radar to estimate how fast the two jetliners were moving. Studies show that both planes were traveling well over the federal limits for altitudes below 10,000 feet, The Times said.

The second plane was flying so fast that it was in danger of breaking up in the air as it approached the south tower, Boeing spokeswoman Liz Verdier told the Times.

“These guys exceeded even the emergency dive speed,” Verdier said. “It’s off the chart.”

Two studies have analyzed the speed of the planes, one by the Federal Aviation Administration in consultation with the National Transportation Safety Board and the other by Eduardo Kausel, a professor of civil and environmental engineering at Massachusetts Institute of Technology.

Both found that United Airlines Flight 175, which hit the south tower at 9:02 a.m., was traveling significantly faster than American Airlines Flight 11, which hit the north tower at approximately 8:46 a.m.

The government’s calculations put the speed of the first plane at 494 mph, and the second at 586 mph. The MIT analysis determined the first plane was traveling 429 mph, and the second 537 mph, The Times said.

The south tower collapsed 56 minutes after the crash. The north tower stood for 102 minutes.

Structural engineers have not determined to what extent the speed of the planes affected how long the buildings stood. They are studying many factors, including the point at which each tower was hit.

The south tower was hit between the 78th and 84th floors, while the north tower was struck between the 94th and 99th floors, meaning more weight bore down on the damaged floors in the south tower, The Times said.