Houston (CNN) A week after famed cardiologist Mark Hausknecht was shot to death in broad daylight by a gunman on a bicycle, his loved ones still struggle to comprehend the brutal killing of the Buddhist, dedicated gardener and former Boy Scout leader they knew.

"It is absolutely unimaginable for anyone to have an issue with Mark," said Randall Hulet, a physics professor at Rice University who has been a close friend to Hausknecht and his family over 30 years. "There aren't many people who were as warm and unpretentious as he was."

Hausknecht, 65, whose many patients included former President George H.W. Bush, was gunned down last week on his less than two-mile bike commute to Houston Methodist Hospital, where he'd worked over three decades. Houston police said there is a "high probability" the attack was targeted but emphasized that the motive is still unknown. Though the suspected gunman was recorded on multiple surveillance cameras, he remains at large.

Hausknecht lived in an upscale neighborhood not far from the Texas Medical Center in southwest Houston with his wife, retired emergency room physician Georgia "G.G." Hsieh. They have two adult sons, Matthew and Paul.

New photos obtained by CNN show what appears to be the suspect in the shooting of former President George H.W. Bush's doctor.

Stunned by the murder, friends and neighbors pulled together to canvass the area where police believe the gunman may have fled, encouraging residents to look through their own security camera footage for any images of the suspect. Two frames obtained by CNN show the suspected gunman cycling on Southgate Boulevard near the site of the attack, but it is still unclear whether these are images from before or after the attack.

As the police investigation unfolds, aided by the FBI's Houston office, potential motives remain a mystery. Friends and colleagues said Hausknecht was not one to have enemies and remembered him as a quiet person with a "purposeful" life.

"Mark was such a warm human being. So many people considered themselves to be his best friend, including me. He was a very special person, at the top of his profession and yet so humble. ... He'd treat his fellow doctors and the lawnmower repair guy exactly the same," Hulet said.

Colleague and friend Dr. Neal Kleiman, medical director of the cardiac catheterization lab at the Houston Methodist DeBakey Heart and Vascular Center, echoed the sentiment. "The past couple days everyone has asked me about the George H.W. Bush story. But that's not how we think of Mark -- we thought of him as an excellent physician, a role model. We didn't think of him as a doctor to the stars."

Mark Hausknecht

Houston Chronicle obituary described Hausknecht's impact on the community as a physician and neighbor, saying, "The man who healed so many hearts during his life, has left so many broken by his death."

Friends and colleagues say Hausknecht was knowledgeable about almost anything and cared deeply for those around him.

"The last few years, Mark's pride and joy was his gardening and growing his own food," said Kleiman. He grew vegetables and had a couple of fruit trees in his backyard. "He liked to barbeque -- he made the best salmon I've ever eaten. Going to his house was a treat."

In recent years, Hausknecht embraced Buddhism, Kleiman and Hulet said. He'd take annual trips to an ashram in Taiwan to meditate and study.

"He believed in hard work and keeping his mind sharp," Hulet said. "I'm a physics professor, and I think he read more about physics than I do. And the books he was reading were not popular books, they were difficult literature."

Despite his demanding profession, Hausknecht found time to serve as Scoutmaster for Houston Boy Scout Troop 11 between 1999 and 2003, when his two sons were scouts there.

Kent Johnston, chairman of Troop 11 at the time, remembered Hausknecht as being environmentally conscious, picking up trash whenever he could, even while canoeing on the rivers. Hausknecht and his sons had three canoes, Johnston remembered, including one that could be taken apart and stowed in a car.

Mark Hausknecht loved canoeing and helped lead trips as a Scoutmaster.

While Hausknecht was Scoutmaster, Johnston said they did quite a bit of canoeing. "He was very interested in going to different rivers and streams." In 2002, Hausknecht organized a three-day, 70-mile trip down the Rio Grande. Four boys and three adults were split between four canoes. Most of the time they were pushed along by currents, but on one windy day it was clear that the person in the solo canoe would have to fight the wind by himself all day. Hausknecht volunteered, even though it wore him out, Johnston said, which was typical of him.

When Johnston retired from the troop after 25 years, Hausknecht came to the celebration. "I guess someone told him I was retiring, and Mark showed up. I thought that was very neat," Johnston said. "I'll be wearing my scout uniform to the funeral; the troop will be present in uniform. I'm expecting a large crowd. We probably won't fit in the church."

Hsieh spoke briefly to CNN outside the couple's home on Tuesday, saying she couldn't imagine why anyone would want to kill her husband. Her background as an emergency room physician exposed her to the "devastating consequences" of gun violence, she said, but she never thought that violence would hit so close to home.

Hausknecht's murder is one of 158 in Houston since the start of this year, according to police spokesman Kese Smith. Houston police have described the area where Hausknecht was killed as relatively safe. He was shot and killed during rush hour, in a part of town teeming with medical staff commuting to work.

"When I found out what had happened, I was in a meeting in Chicago," Kleiman said. "I got a text that said: 'I can't text this, it's about Mark Hausknecht.'... I was in tears. I think a lot of people were."

Both Kleiman and Hulet will speak at Hausknecht's memorial service on Saturday.

"If I break down, that's OK," Kleiman said.