What happens after an assassination attempt

The Kennedy assassination devolved into a law-enforcement fiasco. The Federal Bureau of Investigation, the Secret Service, the Dallas police, and others were all arguing with one another. No one was in charge. It wasn’t yet a federal crime to kill the president. Dallas police were investigating Lee Harvey Oswald, and the Dallas County district attorney would have prosecuted any case against him.

In response to that disaster, Congress enacted a law in 1965 (18 U.S.C. 1751) that makes making killing, attempting to kill, kidnapping or assaulting the president, vice president, acting president or certain White House staffers a federal crime. That law doesn’t apply to former presidents or candidates, but the FBI has said it “has assumed the role of the lead federal law enforcement agency" in investigating Saturday’s shooting.

In the Reagan case, the new statute was clear: The FBI would conduct the investigation. Reagan was shot and wounded on Monday, March 30, 1981. I was the first FBI agent on the scene and manager of the investigation. Lessons learned then are being applied in the investigation of Saturday’s attack.

When I arrived in front of the Washington Hilton, my initial statement to the press was simply “The FBI is on the scene." The FBI’s initial statement Saturday was “FBI personnel are on scene in Butler, Pennsylvania." At 10 p.m. ET, an FBI news release confirmed its Pittsburgh Field Office was running the investigation.

In the transition from the initial crisis response into a major case investigation, easy assumptions must be avoided. In the Reagan attack, the shooter was immediately apprehended. In Butler, the Secret Service killed the apparent shooter. The public and many in the media think it is pretty much finished.

Everybody now knows that Reagan’s shooter, John W. Hinckley Jr., was a mentally disturbed young man. But at the time we didn’t know that. Nor did we know he was acting alone. We knew somebody had tried to kill the president of the United States. We had to ask if it was a conspiracy.

The day of the Reagan attack we learned where Mr. Hinckley had been staying. We got a warrant and searched his hotel room. Before touching anything, we filmed and photographed the entire room. Then we dusted for fingerprints in case there was an accomplice. Everything was “by the book." We wanted to have everything covered in case an additional name came up in connection with the shooting. We wanted to be prepared for any allegation of involvement by others.

What we found in Mr. Hinckley’s room was bizarre. On the desk, laid out for us to find, was his whole plan. He left a map of where he was going. He had the morning’s newspaper open to the president’s daily schedule. He circled that Reagan was going to be addressing a labor-union group in the ballroom of the Washington Hilton. Strangest of all was a statement—a letter to the actress Jodie Foster that proclaimed he was committing a historic act, a presidential assassination, to impress her.

In the midst of the current divisive political campaign, it is even more important to resolve any possibility of the involvement of others in conspiracy. Everything must be done properly and in detail to avoid any allegation of a coverup.

Data searches in FBI headquarters and elsewhere will provide more background information about Saturday’s shooter, as they did about Mr. Hinckley. The serial number of the revolver that was taken from Hinckley was provided to the Bureau of Alcohol, Tobacco and Firearms. It quickly determined where Hinckley bought the gun. The ATF undoubtedly is doing the same for the weapon used Saturday. That likely will produce more leads.

The Butler scene was an outdoor rally. Even more than at the Reagan shooting, there are many potential witnesses. Interviewing them all about what they might have seen at the shooting or before it will be a massive and time-consuming effort. In the Reagan investigation, police herded the witnesses into the auditorium where the president had spoken. They were interviewed in a relatively calm atmosphere. Some witnesses saw a lot, and some had seen Mr. Hinckley beforehand. We also needed to identify Secret Service agents who were witnesses or had other firsthand information, a sometimes delicate and sensitive undertaking.

The Secret Service, understandably, becomes institutionally defensive when something goes wrong. In the Hinckley case, we had been providing the Secret Service with the results of our investigation as they came available—i.e., actual copies of the form FD-302s, on which FBI agents memorialized their interviews.

A few days into the investigation, a Secret Service supervisory agent asked me—told me—we would have to change some of our agents’ FD-302s because “they make the service look bad." Before he could continue his pitch, I told him, “That’s something we just don’t do." The FD-302 was used to record what the agent heard or observed. It was potential testimony for court and had to be totally truthful.

We also had to interview the president, who was both a victim and a potential witness. We were already getting pushback from the White House and the Secret Service, who didn’t want this treated as a proper criminal investigation. We did. In part this difference was a matter of culture. The Secret Service was focused on protection, while our focus was investigation.

The two FBI agents chosen to interview Reagan in the hospital were veterans who had suffered gunshot wounds. We thought they could empathize and establish rapport with the president. They did. Mr. Trump, as both victim and potential witness, will have to be interviewed as well—though he will likely tell the agents, as Reagan did, that he has no idea who shot him.

Our weekslong follow-up investigation traced Mr. Hinckley’s history for months preceding the shooting to see if anyone else was involved with him. We determined he had traveled the country, gone to shooting ranges, and indeed was fixated on Ms. Foster. He had planned and committed an assault on the president. He was a mentally deranged man.

While conspiracy theories about the JFK assassination abound to this day, there was never any suggestion of a coverup in the investigation of Reagan’s shooting. Let’s hope the same is true this time around. Reagan believed that God spared his life for a greater purpose. May Mr. Trump find himself similarly inspired.

Mr. Baker is a retired FBI special agent and legal attaché and author of “The Fall of the FBI: How a Once Great Agency Became a Threat to Democracy."