Attorney General Jeff Sessions said Friday the Justice Department was dedicating every available resource to the investigation "and I can tell you this: We will find the person or persons responsible".
"We will prosecute them, him, her, whoever it may be, to the fullest extent of the law", he said at a White House event.
There were also reports that a possible second package addressed to Clapper was discovered at a U.S. mail facility in NY.
Geoffrey Berman, US attorney for Southern District of NY, called Sayoc's conduct "cowardly and reprehensible and totally anathema to our democracy".
MSNBC also reported a picture of President Trump and numerous bumper stickers supporting him were also seen on the van, along with a presidential seal and a photo of Hillary Clinton shouting with a target over her face.
Public records show numerous arrests over the years for domestic violence, theft and other charges, including the alleged bomb threat against a utility company. Records say he threatened to "throw, place, project or discharge" a destructive device.More news: Bollywood heartthrobs Deepika and Ranveer announce wedding date
Criminal records show Sayoc was arrested on multiple other occasions for fraud, tampering with evidence, drug possession and unlawful use of a driver's license. One was intercepted in New York City at a postal facility.
Cesar Sayoc allegedly placed 13 improvised explosive devices in envelopes and addressed them to political figures across the United States, prosecutors said.
The first package was found Monday at the home of billionaire businessman George Soros, a long-time Democrat and Trump critic.
Though none of the improvised explosive devices actually detonated, Wray stressed that "these are not hoax devices". Each listed Debbie Wasserman Schultz, former chairwoman of the Democratic National Committee, as the sender, including misspellings of her last name, the Federal Bureau of Investigation said.
Trump, who held back some of his usual name-calling at a rally in Wisconsin earlier this week, was back to his usual attack lines Friday evening even as he called for an end to the "politics of personal destruction".
The packages stoked nationwide tensions and fears as voters prepared to vote November 6 to determine partisan control of Congress - a campaign both major political parties have described in near-apocalyptic terms. The most recent packages were intercepted on Friday in California, Florida and NY targeting Harris, Booker and former intelligence chief James Clapper, another prominent Trump critic. Maxine Waters was key to cracking the case, FBI Director Christopher Wray said.
FBI Assistant Director William Sweeney said thorough examinations of all the packages were under way at the FBI laboratory in Quantico, Virginia, near Washington DC.