Charlotte, N.C. ? Investigators seeking the cause of a commuter plane crash said Thursday that the aircraft was near its maximum weight and a recently adjusted tail assembly controlling the plane’s lift was moving unusually during the 37 seconds the doomed flight was in the air.

The up-and-down motion began after the plane underwent routine maintenance Monday night and showed up during all eight flights before Wednesday’s crash, said John Goglia, a National Transportation Safety Board member.

The flight data recorder pulled out of the smoldering wreckage confirmed the unusual movement also occurred just before the crash that killed 21 people.

A team of NTSB investigators was sent to the Raytheon Aerospace facility in Ceredo, W.Va., where the maintenance was done.

“We need to know which procedures were followed,” Goglia said.

Goglia said the plane was about 100 pounds below its maximum weight at takeoff. He said pilots, mechanics, gate agents and baggage handlers interviewed by investigators said they thought the plane “looked heavy.”

“We have enough concern that we’re going to follow that to its conclusion,” Goglia said. “Heavy is not a quantifiable word. We’re going to quantify it.”

Goglia said there was confusion among workers loading the plane on whether too many bags had been put in the luggage compartment near the tail of the plane. After consulting with the captain, however, it was agreed that the plane could handle the extra load.

The Federal Aviation Administration also told Air Midwest officials to check more than 40 planes that may have been serviced at the West Virginia facility. Air Midwest, a commuter airline of the Mesa Group, operates as US Airways Express in some areas.

“It’s pretty clear that Air Midwest needs to take immediate action,” FAA spokesman Greg Martin said.

In a statement, Air Midwest said it had begun immediate inspection of the elevator controls on three aircraft that have undergone similar maintenance in West Virginia. The airline said the inspections will be done by today.