One of Venezuela's most powerful leaders may have put out an order to kill Florida Sen. Marco Rubio, a fervent critic of the South American country's government, according to intelligence obtained by the U.S. last month.

Though federal authorities couldn't be sure at the time if the uncorroborated threat was real, they took it seriously enough that Rubio has been guarded by a security detail for several weeks in both Washington and Miami.

Believed to be behind the order: Diosdado Cabello, the influential former military chief and lawmaker from the ruling socialist party who has publicly feuded with Rubio.

At a July 19 Senate hearing, the same day he was first spotted with more security, Rubio repeated his line that Cabello — who has long been suspected by U.S. authorities of drug trafficking — is "the Pablo Escobar of Venezuela." A week ago on Twitter, Cabello dubbed the senator "Narco Rubio."

The death threat was outlined in a memo to several law enforcement agencies disseminated last month by the Department of Homeland Security. The memo, designated "law enforcement sensitive" but not classified, was obtained by the Miami Herald.

The memo revealed an "order to have Senator Rubio assassinated," though it also warned that "no specific information regarding an assassination plot against Senator Rubio has been garnered thus far" and that the United States had not been able to verify the threat. That Cabello has been a vocal Rubio critic in Venezuelan media was also noted, a sign federal authorities are well aware of the political bluster complicating the situation.

According to the memo, Cabello might have gone as far as to contact "unspecified Mexican nationals" in connection with his plan to harm Rubio.

The United States believes Cabello controls all of Venezuela's security forces. Rubio, a Republican, has President Donald Trump's ear on U.S. policy toward Venezuela.

The Venezuelan Embassy in Washington declined to comment Saturday. Venezuela's Ministry of Communication and Information said Sunday it could not respond to media queries until Monday. Messages sent to some of Cabello's email addresses were not immediately returned.

Rubio declined comment through a spokeswoman. His office had previously sent reporters' questions about the security detail to Capitol Police, which did not respond Saturday but has in the past also declined comment.

Capitol Police "is responsible for the security of members of Congress," Homeland Security spokesman David Lapan said in a statement. "It would be inappropriate for DHS to comment on the seriousness of the threat."

Lawmakers have been on heightened alert since a June 14 shooting in Virginia targeted Republican members of Congress practicing baseball. House Majority Whip Steve Scalise of California was critically injured. He was protected by Capitol Police officers — who ultimately killed the shooter — only because he is a member of congressional leadership.

Capitol reporters first noticed police officers trailing Rubio almost a month ago. When he was interviewed last week by Herald news partner WFOR-CBS 4, Rubio's security included at least one Miami-Dade County Police officer. MDPD was one of the law-enforcement agencies asked to help protect Rubio.

Rubio, 46, has led the push for a robust U.S. response against Venezuelan President Nicolás Maduro's government before and after a disputed July 30 vote that elected a new legislative body whose powers supersede all other government branches — including the opposition-held parliament. Rubio has publicly warned Maduro to beware of people in his inner circle who might be looking to betray him.

The White House was succeeding in creating a regional coalition to pressure Maduro until Trump said offhandedly Friday that a "military option" in Venezuela remained possible, despite little to no support in the United States, Venezuela or Latin America for such intervention.

"We're not surprised by threats from the empire, from its chief Trump," Cabello wrote Saturday on Twitter. "In the face of such deranged imperial threats, each person should man their trench. Mine will be next to the people defending the fatherland!"

For years, U.S. authorities have investigated Cabello and other high-ranking Venezuelan government members for suspected drug smuggling, an allegation Cabello has fiercely denied. Earlier this year, the United States accused Vice President Tareck El Aissami of being a drug kingpin and later revealed he had at least $500 million in illicit funds tucked overseas.

Cabello, 54, is a former army lieutenant who was close to the late President Hugo Chávez and fought alongside him in a failed 1992 coup. A former vice president and head of parliament, Cabello is now a delegate to the new all-powerful constituent assembly. He continues to exert vast influence over the ruling United Socialist Party of Venezuela.

Cabello is not, however, among the 30 Venezuelan officials — including Maduro himself — whom the Trump administration has recently placed under financial sanctions for undermining democracy, engaging in corruption and repressing dissent.

Miami Herald staff writers Charles Rabin and Jay Weaver contributed to this report from Miami, and correspondent Jim Wyss contributed from Bogota. McClatchy correspondents William Douglas and Franco Ordoñez contributed from Washington.