An imbalance in power to the engines of a commuter plane is under consideration as the possible cause of the crash that killed former Sen. John Tower and 22 others last week, federal investigators say.

Tower and his daughter Marian, also killed in the crash, were honored at a memorial service Monday in Dallas attended by President Bush and a delegation from Congress. They remembered the 65-year-old Texas Republican as a patriot.The Atlantic Southeast Airlines plane, a new, twin-engine turboprop, went into a nosedive near here Friday, killing everyone on board.

A propeller on the Brazilian-made Embraer 120 may have been in "flat pitch," or angled so that the blades no longer bore into the air, investigators said. The blades' angle is adjusted by the engine power setting.

"We're looking pretty closely at the engines and the propellers to determine if that occurred," said Barry Strauch, a National Transportation Safety Board investigator.

If flat pitch occurs in flight, it can cause a plane to "fall out of the sky," Strauch said.

Witness reports of "squealing" 10 to 20 seconds before the crash, plus the setting of the aircraft's rudder and descriptions of the plane's movement are consistent with the flat-pitch theory, investigators said.