It was already nearly 90 degrees the morning of July 20 as Mark Hausknecht, dressed in blue scrubs, pedaled his bright yellow bike down Main Street toward work at Houston Methodist Hospital, as he did every day. The renowned cardiologist, who previously treated President George H.W. Bush, crossed a busy intersection just blocks from the heart of the Texas Medical Center, one of the largest medical complexes in the world.

It was just then that another cyclist started catching up to him, according to surveillance footage Houston police released Monday.

The rider was clean-shaven, wearing a tan-colored ball cap, a blue polo and khaki shorts, the chosen ensemble of any number of men in Houston on a hot day. But what stood out about him was the olive-green backpack weighing him down. Police would soon describe it as “fully loaded.”

He followed the doctor for a couple more blocks. Just a half-mile from his hospital, at about 8:45 a.m., he sped up on his tail. Then, in front of a hotel, the cyclist passed him, turned around and fired at 65-year-old Hausknecht, Houston police said. He was hit twice in the torso, according to the police.

The fatal seconds — the shooting itself — were not captured on the surveillance images released Monday. The doctor fell to the ground immediately, police said. A private ambulance driver on Main Street saw him lying there, local news station KTRK reported, and paramedics got out and started rendering first aid and CPR. A witness held Hausknecht’s head in his hands until a vacant ambulance arrived.

But Hausknecht was pronounced dead at Ben Taub Hospital, a public trauma hospital.

The shooting death of one of Houston’s most respected heart doctors in broad daylight, in a heavily trafficked area, has left police perplexed. The assailant remains on the loose, the object of a manhunt with no promising leads so far.

On Monday, police released more detailed photos and the surveillance footage that captured Hausknecht’s last ride as the man followed behind him on the bike — footage police obtained from a city bus that happened to pass them both. From the footage and witness accounts, police have described the suspect as a white or Hispanic male, about 30, with glasses and a slender build.

But a motive remains completely unknown.

“Mark never had an enemy,” Kevin Lisman, a physician who worked with Hausknecht for 15 years, told KTRK. “He was very quiet, and very humble. Just the kind of guy who would take care of anybody at the drop of a hat.”

At the time of his death, Hausknecht worked as a cardiologist within Houston Methodist’s internationally respected DeBakey Heart & Vascular Center and helped lead the Houston Cardiovascular Associates. In a statement published in full by KTRK, Houston Methodist described him as a “compassionate physician with a phenomenal bed side manner” who had been practicing for nearly 40 years and whose patients were “so proud to call him their doctor.” He had become well known in 2000 when he treated President George H.W. Bush for an irregular heartbeat, saying at a news conference watched by thousands that he did not suspect the former president had suffered a heart attack, and “I do not think his life is in peril in any way.”

“Mark was a fantastic cardiologist and a good man,” Bush said in a statement Friday. “I will always be grateful for his exceptional, compassionate care. His family is in our prayers.”

Hausknecht’s wife, emergency medicine physician Georgia R. Hsieh, called her husband’s killing “senseless” in a statement, but said what she found truly senseless was public officials’ lack of action addressing gun violence in America. So many had asked what they could do to help, she said. She asked those listening to “use your vote and your voice to stem the tide of this growing public health epidemic.”

A 65-year-old cardiologist, who treated former president George H.W. Bush in 2000, was fatally shot while riding a bike near Texas Medical Center in Houston. (Reuters)

“After spending his adult life saving and prolonging the lives of others, my talented husband, Dr. Mark Hausknecht, had his life prematurely ended,” she wrote.

“‘Senseless’ has become a trite adjective to describe these tragedies,” she continued, “but what IS senseless is the misguided notion that any society with more guns is a safer society. When students cannot go to school without fear, and teachers need to arm themselves, what has this country come to? As a trained emergency medicine physician, I am no stranger to the devastating consequences of both intentional and accidental firearm use. Now my family and I have joined the ranks of the tens of thousands of grieving Americans who lose innocent loved ones every year.”

By Monday a small memorial was set up near the site of Hausknecht’s death, bearing flowers, a Bible and a sign that said, “Gun Violence. Reform Gun Laws. Vote Now! Road For Change,” the Associated Press reported.