The FBI’s own guidelines restrict the deployment of informants to spy on Americans, such as the bureau’s decision to plant a human source among Donald Trump’s presidential campaign aides.

The cautionary regulation is contained in the FBI’s nearly 700-page Domestic Investigations and Operations Guide. The restrictions are prompting national security analysts to say the FBI should have heeded its own rulebook, which encourages alternatives to human spies in any investigation, much less one into a presidential political campaign.

The FBI should have focused, they say, on Russian agents who were meddling in the election by hacking computers and by spewing false information on social media.

Instead, the bureau, during President Obama’s administration, took the momentous step of recruiting a national security academic, Stefan Halper, to spy on Trump associates by striking up what seemed to be innocent professional contacts.

Mr. Halper was a “confidential human source,” an official category of spy that is regulated by the FBI’s domestic investigations directive. The FBI completed an updated document in 2013 and posted online a redacted version in 2016.

Human sources are regulated under a program called “Otherwise Illegal Activity,” or OIA. It is called “otherwise illegal” because spying on Americans would be against the law if, as the policy says, the spying is “engaged in by a person acting without authorization.”

Thus, the protocol says the confidential informant must be approved by the Justice Department, meaning an Obama political appointee might have given the go-ahead in summer 2016.

The guideline says a human source should be used only in “limited circumstances,” which includes “when that information or evidence is not reasonably available without participation in the OIA.”

The rules also say that “otherwise illegal activity” should be “limited or minimized in scope to only that which is reasonably necessary.”

A U.S. official told The Washington Times that the bureau should have targeted Russian intelligence officials first to determine whether there was evidence that they were contacting or colluding with Trump people before authorizing domestic spying by what the source called an “agent provocateur.”

John Dowd, President Trump’s former defense counsel, said the FBI had a duty to notify, not spy on, Trump people.