An Illinois man accused of sending letters containing suspicious white powder to four members of Congress has been arrested after he allegedly wrote his own name and return address on the envelopes, according to authorities.

Matthew Unglesbee, 36, of Quincy, remained in the custody of US marshals on Tuesday on federal charges of perpetrating a hoax and sending threatening communications through the mail.

In an eight-page criminal complaint unsealed on Monday, Charis Hobbs – a taskforce agent with the FBI’s Springfield office – said Unglesbee admitted during an interview on Sunday that he sent the letters to the four unidentified politicians.

Unglesbee told the agent that the substance he placed inside the envelopes was salt – because it “kind of looks like” anthrax, the complaint said. Preliminary testing did not reveal the presence of any biological or chemical toxin.

“Each letter contained a white granular substance and a handwritten letter which read ‘ha ha’,” the court filing said. A letter to an unnamed US senator also contained granules and read “Hah enjoy the gift”.

On each of the four white, letter-sized envelopes, the agent said, Unglesbee wrote his name and home address in Quincy.

None of the staffers who handled the letters at any of the four Congress members’ offices in northern Illinois opened the envelopes, the complaint said. The offices had alerted each other to look out for mailings with Unglesbee’s name on them.

FBI personnel “trained in handling weapons of mass destruction, and threats of biochemical toxins” were sent to three of the politicians’ offices on Wednesday after staff called authorities, and hazardous materials (hazmat) response teams attended two of them.

Having retrieved the communications with Unglesbee’s address on them, FBI agents visited his home, where they discovered another envelope – empty and addressed, but not sent, to one of the same Congress member’s Washington DC office.

The suspect told investigators that he mailed four letters “after hearing someone in a restaurant mention the news story of other letters being sent”, the filing said. Earlier in October, the FBI and US Postal Inspection Service said they were investigating multiple reports of suspicious letters sent to state officials and election offices across the country.

Unglesbee denied participating in the other suspicious mailings. But he allegedly told investigators that he anticipated the Congress members would “take it serious” because the powder he sent could be perceived to be anthrax, which he said he knew was dangerous and would cause hazmat teams to have to respond.

“Unglesbee claimed he was joking but recognized the recipients would not know he was joking and could perceive it as a threat,” the complaint said.

In a justice department press release on Monday announcing Unglesbee’s arrest, Ryan Presley, special agent in charge of the FBI’s Springfield field office, said authorities were focused on “keeping our public institutions safe from intimidation”.

He said: “Mailing unknown substances to congressional offices creates fear and disrupts the public institutions vital to our democracy. We, along with our partners, will remain vigilant in identifying and holding accountable those whose actions threaten to intimidate public officials or disrupt our democratic systems.”

Unglesbee was tentatively scheduled to appear for a detention hearing on Wednesday afternoon in federal court in Springfield.