(CNN) -- Why Butte?
Martha Guidoni says this photo was taken after she and her husband saw the plane "nosedive" in Butte, Montana.
Of the many unanswered questions surrounding a weekend plane crash that killed the pilot and three families headed for a Montana ski vacation, the first one investigators are examining is why the pilot asked for permission to land in Butte, about 80 miles short of their planned destination, the National Transportation Safety Board said Tuesday.
"It's a question," Mark Rosenker, the NTSB's acting chairman, told reporters. "There's a lot of questions, but it begins with that question."
Rosenker said investigators have no theories as to what caused Sunday's crash, which killed all 14 aboard the single-engine Pilatus PC-12. But the diversion to Butte is "where we're beginning to look at things," he said
The pilot, Ellison "Bud" Summerfield, did not declare any emergency, and Rosenker said Summerfield's voice betrayed no sign of stress when he asked for clearance to divert the aircraft from Bozeman to Butte, a longtime mining town in southwestern Montana.
Investigators have sent a team to Salt Lake City, Utah, to retrieve air traffic control recordings that may shed some light on the decision, NTSB investigator Keith Holloway said.
The plane was owned by an Oregon-based investment firm. The passengers included two daughters of the firm's president, their husbands, another couple and seven children ranging in ages from 2 to 9 from the three families. Watch friends recall crash victims »
The aircraft slammed into a cemetery about 500 feet short of the Butte airport. See a map of the crash site »
Federal aviation regulators warned in early March about possible problems with the controls on the Swiss-built PC-12. The issue stemmed from a cable clamp linking the aircraft's controls to the cable that adjusts the elevator, which controls the plane's pitch.
"This condition, if not corrected, may reduce the effectiveness of the stick-pusher and/or limit elevator control movement," a March 10 advisory from the Federal Aviation Administration states.
Holloway said the NTSB already has examined that issue and determined it had nothing to do with Sunday's crash. But Rosenker said investigators have few other clues.
Private aircraft have no requirement for a cockpit voice recorder or a flight data recorder, and electronic systems aboard the planes aren't built to withstand crashes. Early speculation focused on the fact that there were 14 people aboard the 10-seat aircraft, but Rosenker said Monday that young children often travel on the laps of adult passengers, and he urged reporters not to rush to judgment.
NTSB officials flew in a PC-12 on Tuesday and followed the approach the doomed plane was taking when it went down, Rosenker said. He said investigators may have to subpoena mobile phone records of the passengers before the investigation is complete. But it's "very, very rare" that the agency can't determine the probable cause of a crash, he said.
"Nothing is off the table in this investigation," he said. "But nothing also, at the same time, is leading us to specific working theories."
CNN's Ted Rowlands and Paul Vercammen contributed to this report.