WASHINGTON (AP) — California scientists are hunting ways to predict who’s about to have a heart attack, and they say they’ve found a key clue.

Most heart attacks happen when fatty deposits in an artery burst open and a blood clot forms over the damage, blocking blood flow. Today’s tests can’t tell when that’s about to happen.

On Wednesday, researchers at Scripps Translational Science Institute report they found deformed cells floating in the blood of 50 people who’d just had a heart attack, cells that had flaked off the lining of arteries. The study couldn’t determine how early those cells appear before a heart attack. To start finding out, the researchers developed a blood test they plan to study in people with chest pain.

The study appears in the journal Science Translational Medicine.