New York (CNN) As the United States reinstated economic sanctions on Iran on Monday, American banks were gearing up for retaliatory Iranian cyberattacks.

Bank executives believe Iranian hackers could attempt to disrupt financial services, perhaps as they did between 2011 and 2013 -- with denial-of-service attacks that interrupted bank websites and other internet financial services.

Last week, CNN got rare access to a major American bank's highly guarded cybersecurity defense center in New York, where monitor screens listed "Iranian hackers" as the top "trending threat" at the moment. (North Korea ranked closely behind.) The bank's top cybersecurity executive said his team is bracing for a reprisal, because Iran "might lash out" as a result of the reimposed sanctions.

The bank requested not to be identified, citing a concern that Iran would single it out and direct hackers to attack the institution. The industry as a whole is already on guard, the bank said.

United States Cyber Command, the hacking force within the Department of Defense, said it is working along with other government efforts to counter "malicious cyber activity."

"Iran, while more limited (than some other countries) in the sophistication of their cyber capabilities, (has) demonstrated a greater willingness to conduct destructive cyberattacks that are well beyond the norms of state behavior in peacetime," Lt. Col. Audricia Harris told CNN in an email.

Companies in the private sector may experience attacks first. Two former federal government officials and several cybersecurity experts confirmed to CNN that major American banks are concerned about Iranian retaliation, though the banks themselves declined to comment.

This week, the Trump administration reimposed all the economic penalties that the Obama administration lifted as part of the 2015 Iranian nuclear deal. The reinstated sanctions added almost 700 targets to the US sanctions list, including 50 Iranian financial institutions, making it harder for these targets to engage in business worldwide. The Iranian government has made public statements about its intention to defy US sanctions.

An industry of private security contractors has grown up to protect major banks and other companies from hackers. They have recently warned their clients to heighten their defenses.

"Banks are taking a hard look at Iranian threat actors right now. We've advised all of our clients in the critical infrastructure space to consider the historic hostile actions of Iranian actors given this new development," said John Hultquist, director of research at FireEye, a cybersecurity firm that provides services to major banks.

The banking industry's privately run group that coordinates defenses against cyberattacks -- the Financial Services Information Sharing and Analysis Center -- told CNN it has not seen Iran prepare to attack yet.

"There's no evidence I've seen that the threat is imminent," FS-ISAC CEO Bill Nelson told CNN on Wednesday. "But we were able to effectively defend and respond in 2012. We're certainly better off than we were then."

According to public accusations made two years ago by the Department of Justice, Iran launched massive cyberattacks against the US banking sector from 2011 until 2013. Iranian hackers flooded American financial institutions with garbage computer traffic, jamming the banks' internet services. The attacks started in December 2011 then ramped up in September 2012, when it significantly disrupted customer access to the websites of Bank of America, JPMorgan Chase, Wells Fargo, US Bank and PNC Bank.

At the time, the cyberattack was the largest ever. The cybersecurity firm CrowdStrike called it "unprecedented." The Obama administration's DOJ later indicted seven Iranians who allegedly conducted the cyberattack while working at two companies "on behalf" of the Islamic Revolutionary Guard Corps.

Joshua Motta saw those attacks firsthand while he worked at Cloudflare, an internet service that protects websites from these kinds of aggressive floods of traffic. At the time, he said, "the industry was unprepared." He's now CEO of the cyberinsurance company Coalition, which provides coverage for small businesses like credit unions and regional banks. Motta said they should be preparing for Iran's vengeance.

"This is on everyone's radar. It is almost with 100% certainty the hacking campaign will resume after sanctions," Motta told CNN this week. "And I don't think it's going to be limited to banks." Energy infrastructure, for example, could also come under threat, as it did from Russia earlier this year , according to the Department of Homeland Security.

If Iran does attack the United States, corporations will be faced with hackers whose skills have grown since 2012, according to CrowdStrike.

"They've gotten better in the last six years at intrusion activity. The experience they've gained -- launching wave after wave of destructive attacks against Saudi Arabia -- has helped them increase their capabilities. They are much more formidable," said Dmitri Alperovitch, CrowdStrike's co-founder and chief technology officer.

Since the 2011-2013 Iranian cyberattacks, the US military has taken a more aggressive position against such threats -- one that could result in an immediate counterstrike against Iran.

As CNN reported in September , the US military has been given more authority to launch preventative cyberattacks. A classified Obama administration directive that forced American government hackers to seek the president's approval before launching retaliatory cyberstrikes deemed to have "significant consequences" was replaced by the Trump administration's National Security Presidential Memorandum 13, or NSPM 13.

President Trump's directive, which came into effect September 20, "effectively reversed those restraints" put into place by the previous President, White House national security adviser John Bolton told reporters at the time . "Our hands are not tied as they were in the Obama administration," he said.

"It is now the policy of the United States to shoot back to deter (an) adversary's cyber operations. If we could get into their network, we could destroy their computers. We could take down the Islamic Revolutionary Guard Corps network affecting the banks," said Jason Healey, a senior research scholar at Columbia University's School for International and Public Affairs who is working on a paper about the more aggressive policy and its potential consequences.

This more aggressive cyberwarfare policy carries a risk of escalating tensions, Healey and other experts said. But Healey acknowledged the tensions with Iran are there already.

US Cyber Command, the military's hackers, "want to get in there and grapple, get close and throw elbows, make the adversaries fight for their computer infrastructure," Healey said.