Investigators with the National Transportation Safety Board are about finished sifting through the wreckage of the tour helicopter that crashed one week ago today near Lake Mead. But NTSB personnel are also probing other sources in an effort to determine what caused the crash, which killed two tourist couples and the Las Vegas-based pilot. And there’s something curious in the NTSB’s preliminary report released yesterday: the helicopter deviated from its normal route just prior to the accident.

Data from radar records indicate that the chopper, owned by Las Vegas-based Sundance Helicopters, made some unusual left turns and a sudden ascent about a minute before crashing in a deep ravine. Officials have confirmed that those maneuvers were not part of the standard flight path of the helicopter. Although inflight loss of control is suspected, there may never be conclusive evidence to back that up: the tour helicopter was not equipped with a “black box” data recorder because it was not required to have one.

The report also noted that the aircraft underwent routine maintenance on December 6, the day before its ill-fated flight, during which the tail and main rotor servo-actuators were replaced as was the engine. The chopper also flew one test flight and two tourist flights before the fatal crash.

A couple from Kansas and another from India lost their lives in the tragic incident. Yesterday in a Nevada court, the family of the Indian couple filed a wrongful death lawsuit against Sundance Helicopters. Presumably, the family of the husband and wife from Kansas will do the same in the future.