A U.S. government website has been hacked by people connected to the government of Iran.

The website is the Federal Depository Library Program (fdlp.gov), which is free service that makes federal government publications available.

The website was taken offline in response to the hack. The cyberattack had a pro-Iranian and anti-Trump message.

The message read: “In the name of god. Hacked By Iran Cyber Security Group HackerS. This is only small part of Iran’s cyber ability! We’re always ready.”

Security experts warned that cyber attacks could be part of Iran’s retaliation for the U.S. airstrike on Friday that killed Revolutionary Guard General Qassem Soleimani.

These hackers are backed by the government of Iran. They are among the world’s most aggressive hackers.

Here’s what the hackers did to Trump’s image:

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The hackers can reportedly inject malware that triggers major disruptions to the U.S. public and private sector.

“Potential targets include manufacturing facilities, oil and gas plants and transit systems,” Daily Mail reported.

“A top U.S. cybersecurity official is warning businesses and government agencies to be extra vigilant,” the report added.

More from theDaily Mail:

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It followed the similar hacking of websites for a number of obscure, non-governmental entities, including the Sierra Leone Commercial Bank, the Taiwan Lung Meng Technology Company, and the Human Rights Protection Association of India.

The website for a British company called Bigways was also struck in the cyber attacks….

In 2012 and 2013, in response to U.S. sanctions, Iranian state-backed hackers carried out a series of disruptive denial-of-service attacks that knocked offline the websites of major U.S. banks including Bank of America as well as the New York Stock Exchange and NASDAQ.

Two years later, they wiped servers at the Sands Casino in Las Vegas, crippling hotel and gambling operations.

The destructive attacks on U.S. targets ebbed when Tehran reached a nuclear deal with the Obama administration in 2015.

The killing early Friday in Iraq of Quds Force commander Soleimani – long after Trump scrapped the nuclear deal – completely alters the equation.

‘Our concern is essentially that things are going to go back to the way they were before the agreement,’ said John Hultquist, director of intelligence analysis at the cybersecurity firm FireEye. ‘There are opportunities for them to cause real disruption and destruction.’

Iran has been doing a lot of probing of critical U.S. industrial systems in recent years – trying to gain access – but has limited its destructive attacks to targets in the Middle East, experts say.

It’s not known whether Iranian cyberagents have planted destructive payloads in U.S. infrastructure that could now be triggered.

‘It’s certainly possible,’ Hultquist said. ‘But we haven´t actually seen it.’

Here’s a screen-capture of the message: