An armed man who three incendiary devices at an immigration jail in Washington statehas been found dead after police opened fire, authorities say.

Authorities, who have not released the attacker’s name, are still trying to determine a motive for the early morning firebomb assault, unsure if he intended to destroy the facility or the inmates held inside.

Tacoma police say officers responded at about 4am on Saturday to the privately-run Homeland Security facility that holds migrants pending deportation proceedings.

The centre has also held immigration-seeking parents separated from their children under President Donald Trump’s efforts to deter illegal immigration.

The shooting took place about six hours after a peaceful rally in front of the detention centre, police spokeswoman Loretta Cool said.

She said another rally was planned for later Saturday, but it would have to be held in a different area because of the investigation into the shooting.

The sprawling Tacoma centre is home to hundreds of migrants the Trump administration aims to deport. Photo: ICE

Police said the man set a vehicle on fire and that he attempted to ignite a propane tank to set buildings on fire. Police said that besides the rifle, he had a satchel and flares.

Police said officers called out to the man, and shots were fired by four officers.

The officers weren’t wearing body cameras, but the area is covered by surveillance cameras from the detention centre. She said she didn’t know if the man fired at the officers.

After the gunfire, officers took cover, contained the area and set up medical aid a short distance away, police said.
Officers then located the man and determined he had been shot and was dead at the scene.

-with AAP