The Arizona sheriff leading the investigation into Nancy Guthrie’s disappearance said he believes authorities understand why the 84-year-old was “targeted,” while warning that the suspect could “absolutely” strike again.

Pima County Sheriff Chris Nanos, who has faced criticism over the unsuccessful month-long search for Guthrie, shared the update but declined to reveal further details, noting that others may still be at risk.

“We believe we know why he did this, and we believe it was targeted, but we’re not 100% certain,” Nanos told NBC News on Thursday night.

“Because of that, it would be irresponsible to tell people they’re safe or that they’re not his target,” he added. “Just because it happened to the Guthrie family doesn’t mean others are safe. People should remain alert.”

The sheriff’s office has also been criticized for failing to properly secure the area outside Guthrie’s Tucson home, where reporters — and even a pizza delivery driver — were able to approach and knock on the front door.

Authorities have yet to identify a suspect in the five weeks since Guthrie was abducted, further fueling public backlash over the investigation.