Investigators say it is too soon to say what caused a plane crash that killed a former senator and four others in the U.S. state of Alaska Monday.
Former U.S. Senator Ted Stevens was among those killed in the crash of a small plane near a remote fishing village. Among the four survivors is Sean O'Keefe, a former administrator of the U.S. space agency, NASA. He is reported to be in critical condition at an Anchorage hospital.
In a news conference Friday, National Transportation Safety Board Chairwoman Deborah Hersman said the plane was equipped with an alert system to warn pilots of dangerous terrain. She said it was not clear whether the system was operational at the time of the crash.
She added an emergency locator transmitter was also registered to the aircraft, but did not broadcast its signal.
She said the signal could have launched rescue efforts earlier, and it was unclear why it did not activate.
Weather has not been ruled out as a factor in the crash. Rain, wind and overcast skies were reported in the area Monday, and poor weather conditions have hampered the investigation since.
She says investigators have interviewed first responders to the scene of the crash, a mountainous area 525 kilometers southwest of Anchorage.
Officials say rescue crews did not reach the site where the single-engine propeller aircraft went down until dawn Tuesday because of weather conditions and the remote, rugged location.
Air travel is common within the state of Alaska, because it has large expanses of wilderness and areas not accessible by roads.
U.S. President Barack Obama issued a statement Tuesday praising Stevens and extending condolences to his family and the families of the others who died in the crash.
The 86-year-old Stevens spent 40 years representing Alaska in the U.S. Senate. He was defeated in 2008 following his conviction for failing to report corporate gifts. The conviction was later overturned because of misconduct by federal prosecutors.
In 1978, Stevens was a passenger in a plane crash at the Anchorage airport that killed his first wife, Ann.
Some information for this report was provided by AP, AFP and Reuters.