AVENTURA, Fla. — On Twitter, Cesar Sayoc Jr. lashed out at immigrants, gun control advocates, and prominent Democratic politicians. On Facebook, he misspelled a racial epithet, directing it at the likes of Oprah Winfrey and former President Barack Obama.

With fury in his fingers, he shared inflammatory news stories from Breitbart, hard-edge videos from Fox News, and angry posts from pages like “Handcuffs for Hillary.” He tweeted a threat to former Vice President Joe Biden. And he posted photographs of himself wearing a red “Make America Great Again” hat at one of President Trump’s campaign rallies.

After a frenzied nationwide search for the person who sent 13 makeshift bombs to some of Mr. Trump’s most prominent critics, Mr. Sayoc, 56, was arrested Friday morning in Plantation, Fla., at an AutoZone car parts shop. Authorities released a photograph of a man with a buzz cut and a mouth that drooped toward a frown. They hauled away a white van plastered with bombastic stickers expressing support for Mr. Trump and animosity toward those who clashed with him.

“Dishonest Media,” read one on the van’s back right window. “CNN Sucks.” Cross hairs appeared on a photograph of one of the liberal commentators at the network, which received more than one package from Mr. Sayoc at its offices in New York.