Mr. Aboulafia said the changes appeared to include the addition of special coatings to the skin to absorb radar beams and the replacement of sharp edges on the helicopter with curved ones. The gentler curves could scatter the reflections of other radar beams in too many directions for an air-defense system to put together a coherent picture of the plane, he said.

Bill Sweetman, the editor of a military trade publication owned by Aviation Week, reported that the damaged helicopter appeared to have five or six blades in its tail rotor, instead of the four in a standard Black Hawk. That could have allowed operators to slow the rotor speed and reduce the familiar chop-chop sound that most helicopters make.

A cover on the rotor that looks like a dishpan or a hubcap in the news photographs may have also helped reduce so-called radar signature of the craft, the analysts said.

Lawmakers who were briefed on the mission said the damaged helicopter had not malfunctioned, as initially described by senior administration officials. Instead, they said, it got caught in an air vortex caused by higher-than-expected temperatures and the high compound walls, which blocked the downwash of the rotor blades.

As a result, the helicopter lost its lift power while hovering over the yard and had to make a hard landing, clipping one of the walls with its tail. Some of the Seal members later tried to destroy the craft, presumably to hide the secret stealth components, before boarding larger backup helicopters that carried them to Afghanistan.

Mr. Aboulafia and Mr. Sweetman both said it was harder to quiet a helicopter than a winged plane, given all the whirling blades.