ATLANTA — Friday’s acquittal of Noor Salman, the widow of the gunman who killed 49 people at an Orlando, Fla., nightclub in 2016, on charges that she helped carry out and cover up the attack, likely means that no one will ever be convicted in connection with the siege.

But the scrutiny of Ms. Salman by law enforcement was not unusual.

(Read about Noor Salman’s acquittal.)

In major cases, even when the suspects are killed and never brought to trial, law enforcement officials carry out exhaustive inquiries that can lead to charges against family members and acquaintances. The charges are often, but not always, connected to the crimes that provoked the primary investigation.

“It’s easy to identify people you can charge,” said William N. Nettles, who was the United States attorney in South Carolina when nine people were killed at a church in Charleston. “The hard part is exercising the discretion and charging the right people.”