The federal agencies that respond to airline hijackings have failed in a series of recent incidents to disseminate critical information despite changes in the emergency response system after the terrorist attacks of Sept. 11, 2001.

The Federal Aviation Administration set up secure phone lines at all of its facilities nationwide to enable officials to immediately contact five other agencies -- the Secret Service, FBI, Transportation Security Administration, Bureau of Customs and Border Protection and North American Aerospace Defense Command (NORAD) -- without having to dial numbers.

But a recent series of false alarms showed that once the network was activated information failed to flow rapidly to officials in the field. As a result, authorities responded slowly, sparked confusion or overreacted in the handling of at least four incidents in the past year, including one involving an international airline flight. The latest misstep occurred June 9 when a plane carrying the governor of Kentucky entered Washington airspace for Ronald Reagan's funeral, surprising the TSA and prompting the evacuation of Capitol buildings.

"As these imperfections surface, they are cause for immediate action in the post-9/11 environment," said Rep. Christopher Cox (R-Calif.), chairman of the House Select Committee on Homeland Security, which is investigating the June 9 incident. Bungled responses "can distract us from real threats elsewhere," Cox said. "My strong sense is that the improvements are underway, but this is going to be forever a work in progress."

The House Transportation and Infrastructure Committee's aviation subcommittee plans a hearing Thursday "to evaluate the response of our federal government -- not just the FAA," in hijacking incidents, a spokesman said.

On May 3, FAA officials believed early on that a Singapore Airlines jumbo jet was in no real danger after it began transmitting a discreet hijacking-in-progress code five hours before its scheduled landing at Los Angeles International Airport. The officials passed the information to the headquarters of the federal agencies that respond to hijackings. But it took more than four hours for the report to reach airport police and federal agents in Los Angeles. When the plane landed, armed police rushed aboard only to learn that the pilot had accidentally pushed a wrong button causing the faulty transmission, several sources involved with the incident said.

"Given the events of September 11th, the close coordination between agencies and existence and use of a tight notification process is critical," Los Angeles Mayor James K. Hahn wrote in a letter to Transportation Secretary Norman Y. Mineta. The May 4 Singapore Airlines incident, he said was "a very poor example of that."

Officials at the TSA, the lead agency in charge of air security, knew in Washington for hours about the possible hijacking aboard Singapore Airlines Flight 20 but failed to inform its Los Angeles-based security director, who learned of it from airport officials. The Los Angeles office of the FBI, which is in charge of a hijacking response once a plane is on the ground, did not learn of the incident until the plane had landed.

"I know that next time we'll be prepared," said Special Agent Herb Brown of the FBI in Los Angeles. "These communication glitches will not happen."

A TSA spokesman said the agency has since changed its policy so that its headquarters office immediately informs local officials of all security-related incidents, including those deemed likely to be false alarms.

"We have made significant improvements in airspace security by building in a number of layers to safeguard against the kind of hijacking that occurred on September 11th," said Brian Roehrkasse, a Homeland Security Department spokesman. "But as with any Homeland Security program, we will continue to fine-tune our defenses to ensure we are working as smartly and efficiently as possible."

Still, lapses in communication continue. Last summer, a local pilot was intercepted by a military jet when he was flying in restricted airspace over the Washington area under a federal waiver. The FAA said the pilot was in contact with air traffic controllers but NORAD, which dispatched the jet, was unaware of those communications. Officials have set up a more intensive multi-layered defense for the Washington area, which is surrounded by a 16-mile radius no-fly-zone and an even larger buffer, extending to 50 miles west of Washington, within which all aircraft must stay in radio contact with and identify themselves to ground controllers.

A month ago, air traffic controllers identified a suspicious plane flying toward New York at 16,000 feet in a zigzag pattern. When a New York FAA operations manager Benedict Sliney called NORAD to report the aircraft, a NORAD officer asked whether the FAA was requesting that fighter jets scramble.

"I was unsure whether I had that authority" to request the scramble, Sliney told the 9/11 commission. He was the FAA official who had prohibited takeoffs during the turmoil on Sept. 11, 2001. "I don't believe the lines of communication were as clear as they should be" in the June 4 incident, he said. The plane turned out to be on a photo shoot.

An FAA spokeswoman said its role is to inform NORAD of suspicious planes but it does not suggest whether the military should scramble jets.

The FAA blamed the mix-up involving the Kentucky Gov. Ernie Fletcher's plane on a contractor who was unaware than the military could not identify the governor's plane on its radar. The plane had faulty transmission equipment that made it difficult to be identified. As a result, the TSA and the Capitol Police believed the plane was hostile and called for an evacuation.

"All of these incidents involve communication between human beings," said Laura Brown, an FAA spokeswoman. "We try to make it work perfectly but human beings are not perfect."

Federal officials said that immediately after the 9/11 attacks they began to improve the response to aviation emergencies by updating hijacking training for flight crews and air traffic controllers, and installing the secure phone lines at agencies nationwide.

The 24-hour Domestic Events Net line is a major improvement on protocols in effect on 9/11, when some FAA officials waited to hear from Washington headquarters before making decisions and other authorities used to out-of-date phone numbers in an effort to reach NORAD.

Now, air traffic controllers are constantly on the lookout for suspicious activity. NORAD, which had been focused on defending the country against a Soviet missile attack, has new radar capability to monitor U.S. airspace in tandem with the FAA. NORAD said it has scrambled jets or diverted aircraft more than 1,500 times since the 2001 attacks.

NORAD said it decides whether to intercept suspicious aircraft and does not need an order from the FAA or any other agency to do so.

"NORAD has implemented numerous changes and improvements along with Department of Defense and other government agencies to improve our ability dramatically to respond and defend U.S. airspace," spokesman Joel T. Harper said in a written statement.

Staff writer Spencer Hsu contributed to this report.

Senate Majority Leader Bill Frist is escorted from the Capitol on June 9 when a plane flew into restricted airspace.