SAN FRANCISCO — For decades, Intel was one of the most predictable players in the technology industry. On Thursday, the semiconductor maker blindsided Silicon Valley with the abrupt resignation of its chief executive over a relationship with a subordinate.

The chip company said it was recently informed that Brian Krzanich had a “past consensual relationship” with an Intel employee. An investigation by internal and external counsel then found that Mr. Krzanich, 58, had violated a non-fraternization policy that applies to managers, the company said. So Intel’s board accepted his resignation.

The disclosure about Mr. Krzanich, a soft-spoken chip manufacturing specialist who joined Intel in 1982 and has run it for five years, left many questions unanswered. The company declined to identify the employee involved, when the relationship took place or any additional details. It characterized its internal investigation as “ongoing.”

Mr. Krzanich’s relationship with the subordinate was not recent, said one person briefed on the situation, who declined to be identified because the company discussions over the matter were confidential. Intel found out about the relationship only a few days ago, this person added.