NEW YORK >> It was an accident, not terrorism, but this week’s helicopter crash on the roof of a midtown Manhattan skyscraper has raised serious security concerns because of the ease with which the chopper ventured into one of the nation’s most tightly restricted no-fly zones.

Some of the questions being posed are the same ones that bedeviled authorities after 9/11 nearly two decades ago: Are they able to react quickly enough to a rogue aircraft? What can they realistically do? Is scrambling fighter jets and shooting down the intruder really feasible over densely populated city streets?

Experts say the answers are not so reassuring.

“If someone had bad intent and they took off from the heliport and made a beeline to Trump Tower, none of the good guys are going to get in the air fast enough to stop him,” said Steven Bucci, a retired Army Special Forces officer who help design the post-9/11 system to guard U.S. airspace.

Ever since President Donald Trump’s election in 2016, a one-mile (1.6-kilometer) radius around Trump Tower has been designated National Defense Airspace, one of the highest-level restrictions, requiring express permission from the Federal Aviation Administration for any flights below 3,000 feet (914 meters) and constant radio communication with air-traffic control.

Pilots who don’t adhere to the restriction, according to the FAA, may be “intercepted, detained and interviewed by law enforcement” and “the United States government may use deadly force … if it is determined that the aircraft poses an imminent security threat.”

Investigators say the pilot who died in the crash Monday afternoon just a few blocks from Trump Tower did not seek such permission and didn’t contact air-traffic control because he wasn’t required to do so, given his intended route, which was supposed to take him around Manhattan to the helicopter’s home base in New Jersey.

After taking off from a heliport on Manhattan’s East Side, the chopper instead strayed over midtown in heavy rain and thick fog and slammed into the roof of the 750-foot (229-meter) AXA Equitable building during a flight that lasted 11 minutes.

An official who was briefed on the situation and spoke on condition of anonymity because the federal investigation is still going on said that 58-year-old commercial pilot Tim McCormack radioed just before the crash that he was lost and trying to get back to the heliport.

Whether anyone noticed the plane’s intrusion into the no-fly zone before the crash is unclear. The FAA and National Transportation Safety Board officials said that question is under investigation. The New York Police Department has a squadron of helicopters that patrol the city’s airspace, but none were in the air at the time of the crash.

Mayor Bill de Blasio is among those calling for tighter regulation of helicopter flights over the city.

“I think the FAA needs to look at this very carefully,” he said on CNN. “Do they need to toughen up their rules or put more security or monitoring of the situation to make sure something like this couldn’t happen again?”

The airspace over certain areas — key government buildings and defense installations, for example — has long been off-limits to planes and helicopters. The use of temporary flight restrictions, or TFRs, grew rapidly after the Sept. 11 attacks and came to include bans over major sporting events and areas around presidential visits.

Federal and civilian air officials say it is not unusual for pilots to venture into such areas without permission, and normally it is just a mistake by a pilot who has strayed off course. Air traffic controllers try to reach the pilot by radio and tell the person how to safely leave the area and land at an airport, and pilots usually cooperate.

In the rare cases when the pilot doesn’t answer the radio or disobeys a controller’s instructions, the FAA contacts government agencies such as the Air Force or Homeland Security to intercept the offending aircraft and guide it out of the no-fly zone. If that doesn’t work, authorities may take more drastic action.

On Sept. 11, 2001, fighter jets were scrambled from bases in Virginia and Massachusetts to engage the hijacked passenger planes, but the airliners went down before the jets could get close.

No aircraft has been shot down over the U.S. since 9/11, but there have been many cases that came close. Last August, after a ground crew employee at Seattle-Tacoma International Airport stole an empty plane and started doing aerobatic stunts, F-15 jets took off but did not fire. The plane crashed on a sparsely populated island, killing just the hijacker.

John Desmarais, operations director for the Civil Air Patrol at its headquarters near Montgomery, Alabama, said the decision to shoot down a plane would be complicated in New York City. Firing a missile over the city of 8 million people and blasting a plane to pieces, causing a shower of flaming debris, would itself be extraordinarily dangerous.

“I would not think that they would do that over the city, but it depends on what the threats are and the needs at the time,” Desmarais said. “Everybody is going to do their best, but in the end, it would be a tough decision to have to make no matter where that is.”

From 2016 to 2018, the North American Aerospace Defense Command says, it has responded to about 500 flights flagged by FAA as entering no-fly zones without permission or displaying “erratic” behavior. Of those, NORAD says, the military or local law enforcement sent aircraft on intercept missions 164 times.

NORAD spokesman Cameron Hillier said he did not have figures for the number of intercepts in New York City. But nearby areas in New Jersey reportedly have had plenty. The North Jersey Record reported that aircraft were sent to intercept violators around Trump’s golf course in Bedminster, New Jersey, 29 times in 2017 alone.

Hillier said air combat stations are “spread throughout the U.S. and Canada and are capable of responding to any aircraft,” but he would not disclose the number of fighter jets in the New York City area.

Jeffrey Price, a pilot and aviation security expert at Metropolitan State University of Denver, said military commanders would need “extraordinary, credible information” about a threat before ordering a plane to be shot down.

“Fortunately since 9/11 we’ve never had to come to that conclusion of shoot or don’t shoot,” he said. “To make that decision, there is going to have to be a lot of evidence to say, ‘This thing is a threat and the only way to solve this problem is to put a missile into it.’”