Lindsey Buckingham, longtime guitarist, singer, and one of the key architects behind Fleetwood Mac, was attacked this week in Santa Monica.

According to law enforcement sources, Buckingham showed up for a scheduled appointment when a woman, described as a stalking suspect, approached and threw an unknown substance at him before fleeing the scene. He was not injured in the incident.

Authorities say the woman wasn’t a random stranger. She’s reportedly someone Buckingham, now 76, has dealt with before. Investigators believe she tracked down the time and location of his appointment ahead of time. Police have identified her, and an arrest is expected.

Both Santa Monica police and the LAPD are working the case, while Buckingham’s camp hasn’t publicly commented yet.