According to new legal docs ... the FBI arrested Chase Bliss Colasurdo Wednesday in Washington state after an investigation uncovered multiple serious death threats directed at President Trump's family members.

Authorities were tipped off in March about Colasurdo's alarming social media posts, in which he targeted Kushner and Trump Jr. by name. In February, he also sent an email to 5 different media outlets saying ... "I'm going to personally Execute [Kushner] for his countless treasonous crimes."

Around the same time, he posted a photo on Instagram of what appears to be himself holding a gun and captioned it ... "I made a death threat against [Kushner] yesterday and I have not been arrested yet."

As for Donald Jr. ... Colasurdo posted a photo of him in March and commented ... "I would just like to let the secret service know that I am going to Execute this fa***t."

According to the docs, obtained by TMZ, FBI and Secret Service agents visited Colasurdo on March 19, and he claimed he'd been hacked -- but also stated he was diagnosed with mental health issues years ago.

The feds say his online rantings continued ... in fact, the very next week Colasurdo posted a pic of himself holding a handgun with the caption, "It's Time To Start Bombing Synagogues." He later made reference, in one of his posts, to the San Diego synagogue shooting.

Prosecutors say the FBI noticed in early April, Colasurdo was loading up on weapons, Kevlar body armor and ammo. The feds say after they arrested him on Wednesday ... they searched Colasurdo's home and found a massive cache which included night vision goggles, gas masks, 345 rounds of ammunition and LED laser mounts.

In addition to the gear, agents say they found Nazi flags, a framed portrait of Hitler and books that included, "Hitler's Revolution."

As we reported, law enforcement says Colasurdo also made death threats against Shapiro and his family.