IT was with great trepidation that I approached 3307 N Street in Washington on Nov. 10, 1960. I had just been given the assignment of providing protection for the wife of the newly elected president of the United States, and I was about to meet her for the first time.

I soon realized I had little to worry about. Jacqueline Bouvier Kennedy, just 31 years old at the time, was a gracious woman who put me immediately at ease. She was the first lady, but she was also a caring mother; her daughter, Caroline, was nearly 3 years old, and she was pregnant with her second child. Three weeks later, she went into early labor with John Jr., and I followed her through the entire process. It would be the first of many experiences we would have together.

Being on the first lady’s detail was a lot different from being on the president’s. It was just the two of us, traveling the world together. Mrs. Kennedy was active and energetic — she loved to play tennis, water-ski and ride horses. She had a great sense of humor, and we grew to trust and confide in each other, as close friends do.

In early 1963, Mrs. Kennedy shared with me the happy news that she was pregnant again. She had curtailed her physical activities and had settled into a routine at the Kennedy compound in Hyannis Port, Mass., for the last few months of her pregnancy. I was on a rare day off when I got the call that she had gone into early labor. I raced to the hospital at Otis Air Force Base, arriving shortly after she did.

The president, who had been in Washington, arrived soon after she delivered their new baby boy, whom they named Patrick Bouvier Kennedy.

When Patrick died two days later, Mrs. Kennedy was devastated. I felt as if my own son had died, and we grieved together.

The following weeks were difficult as I watched her fall into a deep depression. Eventually, it was suggested that she needed to get away. In October 1963 I traveled with her to the Mediterranean, where we stayed aboard Aristotle Onassis’ yacht, the Christina. The trip to Greece, Turkey and Yugoslavia, along with a short stop in Morocco, seemed to be good therapy, and by the time we returned to Washington the light had returned to her eyes.

I was surprised, however, when not long after our return Mrs. Kennedy decided to join her husband on his trip to Texas. It was so soon after the loss of her son, and she hadn’t accompanied the president on any domestic political trips since his election.

Nevertheless, when we left the White House on Thursday, Nov. 21, I could tell that Mrs. Kennedy was truly excited. I remember thinking this would be a real test of her recovery, and that if she enjoyed the campaigning it would probably be a regular occurrence as soon as the 1964 race got into full swing.

The first day of the trip was exhausting. We had motorcades in San Antonio, Houston and finally Fort Worth, where we arrived around midnight. It had been a long day for everyone, and Mrs. Kennedy was drained.

On the morning of Nov. 22, I went to her room at the Hotel Texas to bring her down to the breakfast where President John F. Kennedy was speaking. She was refreshed and eager to head to Dallas. She had chosen a pink suit with a matching hat to wear at their many appearances that day, and she looked exquisite.

The motorcade began like any of the many that I had been a part of as an agent — with the adrenaline flowing, the members of the detail on alert. I was riding on the running board of the car just behind the president’s.

We were traveling through Dallas en route to the Trade Mart, where the president was to give a lunchtime speech, when I heard an explosive noise from my right rear. As I turned toward the sound, I scanned the presidential limousine and saw the president grab at his throat and lurch to the left.

I jumped off the running board and ran toward his car. I was so focused on getting to the president and Mrs. Kennedy to provide them cover that I didn’t hear the second shot.

I was just feet away when I heard and felt the effects of a third shot. It hit the president in the upper right rear of his head, and blood was everywhere. Once in the back seat, I threw myself on top of the president and first lady so that if another shot came, it would hit me instead.

The detail went into action. We didn’t stop to think about what happened; our every move and thought went into rushing the president and Mrs. Kennedy to the nearest hospital.

I stayed by Mrs. Kennedy’s side for the next four days. The woman who just a few days before had been so happy and exuberant about this trip to Texas was in deep shock. Her eyes reflected the sorrow of the nation and the world — a sorrow we still feel today.

Clint Hill, a former assistant director of the Secret Service, served under five presidents.