HOUSTON – Authorities have released new images and video showing a suspect sought in the death of a Houston cardiologist who was fatally shot while riding his bicycle in Houston. Dr. Mark Hausknecht was shot to death while riding his bicycle to work Friday morning at Texas Medical Center. Police have said the shooter rode past Hausknecht, then turned around and opened fire.

Authorities released surveillance photos and video that appear to show the alleged shooter riding behind Hausknecht, who was dressed in blue medical scrubs, moments before the fatal encounter. The suspect, police say, was wearing a "blue, short sleeved polo shirt, (not a gray warm-up suit), a khaki ball cap and khaki shorts. He has a large, fully-loaded, olive green backpack."

UPDATE #4 New surveillance photo of the suspect wanted in the killing of Dr. Hausknecht. Suspect is now described as wearing a blue, short sleeved polo shirt, (not a gray warm-up suit), a khaki ball cap and khaki shorts. He has a large, fully-loaded, olive green backpack. pic.twitter.com/VRk66CFeXb — Houston Police (@houstonpolice) July 23, 2018

This is surveillance video from a Metro Lift bus traveling southbound on Main. The doctor and suspect are traveling northbound on Main. No other information at this time. Call HPD Homicide or @CrimeStopHOU if you can ID the suspect. All updates will be posted here. pic.twitter.com/eNwbA4zql6 — Houston Police (@houstonpolice) July 23, 2018

Hausknecht usually rode his bicycle to work each morning, as he lived less than 2 miles from Houston Methodist Hospital.

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He was following his normal routine last week when a man also riding a bike approached the Houston cardiologist from behind and shot him twice in the torso, killing the 65-year-old physician.

Friends, family and patients say they can't imagine that anyone would want to hurt Hausknecht, who once treated former President George H.W. Bush. They say he was a humble and generous man who deeply cared about his patients, volunteered in his community and cared about the environment, often recycling cans on his daily commute.

On Monday, authorities in Houston continued pursuing several leads in Friday's shooting and searching for the unknown assailant. They released a composite sketch over the weekend that showed a man wearing glasses and a cap.

Dr. Neal Kleiman, who knew Hausknecht for more than 30 years, said Monday he remained shocked by the cardiologist's death.

"It's like getting kicked in the gut because it's a friend," said Kleiman, medical director of the cardiac catheterization lab at Houston Methodist DeBakey Heart and Vascular Center.

Kleiman said it's hard to comprehend that the shooting took place in broad daylight at the Texas Medical Center, a 1,345-acre (545-hectare) complex of hospitals and medical institutions that in the morning is busy with traffic, medical professionals heading to work, people going to medical appointments and construction workers.

Hausknecht's wife, Dr. Georgia R. Hsieh, said in a statement that her husband loved Houston and to honor him, people should "work hard, be responsible, patient, tolerant and compassionate."

Hsieh also spoke of the need for gun control legislation, asking people to write, email or tweet their congressional representatives "urging sensible gun laws."

Flowers and a copy of the Bible were placed at a makeshift memorial near where Hausknecht was shot. Someone also left a handwritten poster that read: Gun Violence. Reform Gun Laws. Vote Now! Road For Change.

Kleiman said Hausknecht loved to grow vegetables in his garden and was environmentally conscious, recycling everything.

"He had genuine concern for things around him, including the environment," Kleiman said.

James Pierce, a 43-year-old homeless man who usually saw Hausknecht bike to work each morning but didn't on the day of his death, said he sometimes collected aluminum cans for the cardiologist, who would take them away after work.

Pierce said he didn't know that Hausknecht was a doctor.

Hausknecht treated Bush in February 2000 for an irregular heartbeat after the ex-president complained about lightheadedness while visiting Florida. But it was not something the humble Hausknecht discussed, Kleiman said.

Pierce stood at an intersection near the shooting Monday, holding up a flyer with the shooting suspect sketch printed on it and showing it to passing cars.

"We need to find this guy," Pierce said, hoping someone would recognize the man in the sketch.

He said Hausknecht "heals people."

"Why would somebody want to hurt him?" Pierce said.

Kleiman said Hausknecht also will be remembered for the compassion and kindness that he showed his patients.

"Doctors easily become condescending and snooty," he said. "I don't think that was ever the case with Mark. That kind of approach was just not in his personality."

Jan Case, whose husband was treated by Hausknecht in the early 2000s, still fondly recalls how the cardiologist and her husband would talk about classical music during their appointments.

"He didn't mind spending a few minutes of time to share and be a friend as well as a doctor. He was just a very kind, gentle, man," Case said.