If you refer back to their source there is no mention of antifa at all. The article specifically includes

The man’s identity and his motive remained unknown, police spokeswoman Loretta Cool told Reuters, adding that the medical examiner’s report could be expected as early as Monday.

This recent article from AP says:

A 69-year-old man armed with a rifle threw incendiary devices at an immigration jail in Washington state early Saturday morning, then was found dead after four police officers arrived and opened fire, authorities said.

Again nothing about antifa.

Looks like theepochtimes is living down to its reputation of inventing 'facts' again.