The Federal Bureau of Investigation considered creating an alert system seven years ago to tip off agents to multiple gun purchases like those made by the Orlando, Fla., nightclub gunman, but abandoned the idea over legal concerns, according to multiple officials.

Law enforcement officials who study mass shooters say that multiple firearm purchases can be a key indicator that a troubled individual is taking concrete steps toward violence, much like Orlando shooter Omar Mateen, who bought a handgun and a rifle in the days before he killed 49 people.

The Orlando attack has prompted calls for changes in surveillance and gun control laws, but it isn’t clear any of them would have flagged the gunman.

Democrats and some Republicans have pushed for legislation that would bar or delay gun purchases by people on terror-watch lists, but such measures don’t have enough support to pass either chamber in Congress. Currently, the FBI issues internal alerts when an individual on such a list buys a firearm, but not when someone is no longer on a list.

Separately, FBI and Justice Department officials are evaluating if it would be legal or practical to create an alert system to warn investigators when gun purchases are made by people who, like Mateen, were once investigated for terrorism, according to officials familiar with the discussions.