Federal aviation records show that a week before a fatal helicopter crash, a notice was issued that a tightrope would be stretched across a mountainous area of Arizona, providing a warning for pilots to watch out for the temporary obstacle as they charted flights through the region.

The helicopter pilot and three family members were killed after colliding with that slackline near Telegraph Canyon. Authorities say a witness reported seeing the helicopter strike a portion of the line before falling to the bottom of the canyon.

Investigators with the National Transportation Safety Board will be combing through the wreckage over the coming weeks as they try to determine what led up to the Jan. 2 crash.

The case is making ripples through the aviation world, with safety experts unaware of any other helicopter crashes that have involved a slackline.

Power lines and unmarked cables are a perpetual concern for helicopter pilots because they are inherently difficult to see without markers, lighting or other infrastructure like power poles, said Tom Anthony, director of the Aviation Safety and Security Program at the University of Southern California.

He said it’s important for pilots to review Notices to Airmen, or NOTAMs, while planning their flights. Under FAA regulations, that means checking for any notices that might be issued along an entire flight path, not just where a flight departs or lands.

The Federal Aviation Administration says pilots also should review notices within 25 nautical miles (46 kilometers) of their flight path to be safe. Such notices can warn pilots about anything from slacklines to work along certain runways and what airspace might be restricted.

Anthony acknowledges there can be many notices to review.

“It can be a drudge, a chore, but you’ve got to do it,” he said.

It will be up to federal investigators to determine if there were any mechanical issues or other problems leading up to the crash. It could be months before a final report is issued.

Authorities on Monday confirmed the identities of those who were killed. They were David McCarty, the 59-year-old pilot, and his nieces Rachel McCarty, Faith McCarty and Katelyn Heideman.

McCarty started Columbia Basin Helicopters in the 1990s, building the company into one that was awarded millions of dollars in federal contracts for firefighting support, agricultural spraying, seeding and other projects across nine western states. The company has offices in Oregon and Arizona.

Philip Hofbauer of Ukiah, Oregon, has been friends with the McCarty family since grade school. Later in high school, he worked at their farm near the town of Echo, in eastern Oregon.

“They’re all just amazing people. They’ve all been a huge inspiration in my life,” he said of David McCarty, his siblings and their parents.

Hofbauer told The Associated Press he would see McCarty when he would contract out his heavy equipment to help officials fight forest fires, and McCarty did the same with his helicopters.

Hofbauer said losing four family members is just overwhelming, “a very tragic situation.”

The crash happened about 64 miles (103 kilometers) east of Phoenix, after the helicopter had taken off from an airport in Queen Creek. It took crews hours to travel by foot to the remote area where the helicopter crashed.

Authorities said the slackline was more than half a mile (more than a kilometer) long. The notice warning of the line was issued Dec. 26, noting that it would be up for several days and would be flagged and lit.

According to the Aircraft Owners and Pilots Association Air Safety Institute, on average a few helicopter crashes each year can be attributed to striking wires or towers. However, the institute said FAA charts and apps that help pilots with flight planning, navigation, checklists and real-time data like weather can highlight obstacles.

Hassan Shahidi has been flying for many years. As president and chief executive of the Flight Safety Foundation in Virginia, he said safety revolves around situational awareness and an understanding of the airspace.

“Unfortunately, we’re seeing too many of these incidents where, you know, it’s really preventable,” he said. “It should not happen.”