U.S. Sens. Brian Schatz and Mazie Hirono called for an investigation into the Federal Aviation Administration following a “troubling report” that details complaints alleging the agency ignored serious concerns raised about the safety of tour helicopters in Hawaii prior to two fatal crashes.

“For the past year, we have raised concerns that the FAA has ignored warnings about the safety of aircraft operations. This report exposed troubling new accounts about how the FAA failed to take action on warnings about the safety of helicopters in Hawaii – warnings that could have saved lives,” Schatz said in a statement today. “With more than a dozen helicopter accidents in Hawaii over the last five years, it is clear that we need answers from the FAA and stronger protections to keep people safe.”

CBS News reported an FAA whistleblower claimed his bosses prevented him from conducting aircraft inspections at Safari Helicopters on Kauai prior to the December crash that killed seven people.

Joseph Monfort, an FAA employee since 2009, told Senate investigators that his FAA bosses twice denied him travel authorizations to visit the helicopter company, “making it next to impossible to perform adequate oversight.”

On Dec. 26, an Airbus AS350 B2 helicopter operated by Safari crashed into a cliff face in Kokee, killing the pilot and six passengers.

Another copter crash involving a Robinson R44 sightseeing helicopter operated by Novictor Helicopters crashed on Oneawa Street in Kailua on April 29, killing the pilot and two passengers onboard.

CBS News reported the whistleblower claims “the FAA did not provide adequate oversight that could have prevented the accident.”

The Associated Press reported that a whistleblower told a Senate committee that a manager in the FAA’s Hawaii field office improperly let a helicopter tour company owner certify pilots for flight on behalf of the agency. The owner then approved a pilot, who was at the controls 10 days later in April 29 crash.

Novictor Aviation has been involved in three crashes during the past two years, according to a Jan. 24 letter from Sen. Roger Wicker, a Mississippi Republican and chairman of the Senate Transportation Committee.

Two whistleblowers allege an inappropriately close relationship between FAA managers and Novictor, the AP reported.

An FAA manager, in an interview today with the AP, said the allegation that he improperly granted “check airman” status to Novictor’s owner was incorrect. He accused the whistleblower of “making up stories again.”

Allegations raised by the whistleblowers also include managers directing that investigative reports be altered, and management retaliation against an employee who reported the problems, according to the letter from Wicker to Inspector General Calvin Scovel III of the U.S. Department of Transportation.

The committee says in a fact sheet on the case that its own investigation isn’t complete, but it “raises significant concerns about the efficacy of FAA oversight in Hawaii.”

Hirono said, “In the past year, Hawaii has experienced two tragic accidents involving air tour operators. Safety is a key mission of the FAA, and the whistleblower allegations that the Honolulu Flight Standards District Office knowingly failed to meet this mission deserve immediate action and a thorough investigation.”

Schatz and Hirono sent a letter today to Scovel, requesting an investigation into FAA’s alleged lack of oversight of the safety of tour helicopters in Hawaii.

“The recent air tour tragedies that have occurred in Hawaii have shaken public confidence and raised concerns with the safety of the air tour industry. The U.S. Senate Committee on Commerce, Science, & Transportation has released details of an investigation into helicopter safety which highlights flaws in internal FAA safety culture. The report includes a whistleblower complaint alleging that the FAA directly disregarded serious concerns about the safety of helicopter air tours in Hawaii,” Schatz and Hirono said in the letter.

“We request that you launch an investigation into the specific oversight lapses raised by the whistleblowers as it relates to helicopter operations in the State of Hawaii, as well as policies and procedures within the Office of Aviation Safety, the Western-Pacific Region, and the Hawaii Flight Standards District Offices that may result in increased risk to operators, passengers, and the general public,” they said.

In a statement to the Star-Advertiser today, the FAA said, “The FAA takes allegations of wrongdoing very seriously and prioritizes safety above all else. The U.S. Senate Committee on Commerce, Science, and Transportation acknowledges that its review of these allegations is “incomplete and not yet conclusive.”

“The FAA itself has been investigating these matters and is already taking steps to address substantiated concerns. As we have communicated to the Committee, we cannot comment further on any pending investigations or potential enforcement actions. The FAA will cooperate with any investigation that the Office of the Inspector General might undertake, in the interest of establishing a thorough, fact-based record upon which to base any appropriate corrective action.”

Letter to U.S. DOT Inspector General Calvin Scovel by Honolulu Star-Advertiser on Scribd

The Associated Press contributed to this report.