Washington (CNN) On Sunday, President Donald Trump called on the Justice Department to open an internal investigation into whether the FBI placed an informant into his 2016 presidential campaign to spy on him. By Sunday night, Deputy Attorney General Rod Rosenstein had made clear that Justice would investigate whether any wrongdoing was found in the use of the informant, who, according to CNN sources, was never embedded in the campaign.

How solid is the ground Trump is treading on? What's the difference between a spy and an informant? And how does the FBI go about getting approval for an informant? I asked former FBI special agent Josh Campbell , a CNN contributor, all of these questions -- and more!

His answers, lightly edited for flow, are below.

Cillizza: Let's start basic. Why does the FBI use informants?

Campbell: Informants often serve as the backbone of important investigations, because they take the FBI into places they could never go on their own. If a special agent running a lawfully predicated criminal or counterintelligence investigation wants to learn more about what the targets of the investigation are up to, a logical investigative tool would be the use of an informant who could report back on the inner workings of a network.

While it would be difficult for an FBI special agent to walk into Trump Tower and glean on his or her own whether Russians were attempting to infiltrate the Trump campaign, a confidential informant -- with "placement" and "access," to use the terms of the trade -- could be extremely useful in helping mitigate potential threats to US national security.

Cillizza: What is the approval process -- if any -- for the FBI to get clearance to deploy an informant?

Campbell: While I can't get into specifics due to the sensitivities surrounding the use of human intelligence sources, I'll say from vast experience running informants domestically and overseas that this investigative tool is among the most highly scrutinized within the Department of Justice. Not only are there very specific guidelines surrounding the deployment of sources, each source operation is continually and meticulously reviewed in order to ensure the source is operating lawfully and does not present undue risk to the government.

It's also important to remember that investigations involving political officials receive even greater oversight as they fall within a category deemed Sensitive Investigative Matters, or SIMs. Understanding that certain segments of society -- politicians, the press, clergy, etc. -- should receive special protections in order to prevent any malfeasance on the part of aggressive investigators, operations involving SIMs would require high-level review and approval.

Cillizza: President Trump has cast this informant as a spy. What's the technical difference (if any) between those two terms and jobs?

Campbell: The President casting a human intelligence source as a "spy" is pure politics. Although informants and spies both technically gather information covertly, the word "informant" is generally reserved for someone righteously operating on behalf of law enforcement, whereas "spy" conjures up a more sinister mental picture of someone skulking in the shadows with questionable intentions.

It is interesting that the President's attorney, Rudy Giuliani, has frequently used the term "spy" as part of a coordinated campaign to discredit law enforcement officers investigating the campaign. Mr. Giuliani was once the US attorney in Manhattan, who oversaw countless government investigations utilizing confidential informants. I doubt he ever referred to them as "spies."

Cillizza: What's the danger in Congress knowing this person's identity?

Campbell: Congressional oversight of the US intelligence community has been enshrined in our way of life since the revelation of abuses of power by government agencies in the 1970s. Understanding government agencies should not be free to police themselves, congressional intelligence committees were formed to watch the watchers. Although agencies have been required to keep Congress fully informed of their operations, there has long been a norm that actual sources and methods were off-limits.

In extraordinary circumstances, I could envision government agencies briefing congressional leaders on their sources, but that would require a level of trust that has been lost since the infamous " Nunes memo ," which many in government believe politicized the intelligence process by selectively releasing pieces of information for the purposes of painting a political narrative.

As someone who has sat across from individuals around the world, trying to persuade them to become government informants, I can tell you that the relationship between the government and a source is one based on trust. The government must trust the information the source provides will be accurate, and the source must trust that the government will protect his or her identity from disclosure. That the DOJ would refuse to provide Congress with information on a source because the agency fears it will be leaked, and thereby endangering the source, says a lot about the presently strained relationship between the Department of Justice and its congressional overseers.

Cillizza: Finish this sentence: "The likelihood this informant was placed in Trump's campaign as a way to spy on his operation is _________." Now, explain.

Campbell: "The likelihood this informant was placed in Trump's campaign as a way to spy on his operation is possible, but lawful."

First, it appears as though the FBI was conducting a properly predicated counterintelligence investigation in order to determine whether the government of Russia posed a threat to national security. We do not know whether the initial focus was on sussing out specific operatives from Russia, or on identifying members of the campaign who were already working on Russia's behalf.

Either way, the genesis of such a counterintelligence [operation] would have been scrutinized by the highest levels of the FBI and the Justice Department. Taking the step of using an intrusive investigative tool like a human informant would have garnered even greater oversight. In the face of blistering political attacks from the President, the public should find comfort knowing that the actions of DOJ officials under the Obama administration are now being defended by Justice Department officials appointed by Trump.

As a former FBI special agent, I often focus on what people don't say when offering a defense to allegations of potential wrongdoing. Although there appears to be no evidence to date that the President personally colluded with Russia, it remains interesting to me that the White House legal strategy involving this confidential human informant has been to attack what the informant did rather than what he may have found.