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The three Michigan women and two men who were killed when a pickup truck plowed into their bicycling group were identified Wednesday as fellow bicyclists organized a Ride of Silence in their honor.

An unidentified man who fled the scene was in custody in connection with the incident, which also seriously injured four other people Tuesday night in Cooper Township, north of Kalamazoo.

Kalamazoo County Prosecuting Attorney Jeffrey S. Getting identified the victims Wednesday as:

Debra Ann Bradley, 53, of Augusta, Michigan.

Melissa Ann Fevig-Hughes, 42, of Augusta.

Fred Anton (Tony) Nelson, 73, of Kalamazoo.

Lorenz John (Larry) Paulik, 74, of Kalamazoo.

Suzanne Joan Sippel, 56, of Augusta.

Two of the four injured bicyclists remained in critical condition at Borgess Medical Center, the hospital told NBC News. The two others were in serious condition at Bronson Methodist Hospital.

Investigators at the scene of a fatal crash Tuesday in Cooper Township, Michigan. WOOD-TV via NBC News Channel

Vicki Cessna, a spokeswoman for the Catholic Diocese of Kalamazoo, told NBC News that Nelson and Paulik were "very active" members of St. Thomas More Catholic Parish on the campus of Western Michigan University. Bradley was a member of St. Ann Catholic Church in Augusta, she said.

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Michigan State University said Sippel was a database manager for the Great Lakes Bioenergy Research Center at the university.

"Our thoughts and prayers are with the family and friends of Suzanne during this very difficult time, and our focus is on providing caring support to them," the school said.

Little information about Fevig-Hughes was immediately available.

What we know for certain is that Michiganders as a family are in mourning as #Kalamazoo struggles to understand another senseless tragedy. — Governor Rick Snyder (@onetoughnerd) June 8, 2016

Gettings said Wednesday that charges against the driver, if any, could be filed as early as Thursday. He said the driver wouldn't be identified until then; the Kalamazoo County Jail said it couldn't discuss any inmates in its custody.

"There's an overwhelming sadness for our victims, for the people who were injured, for the families of those who were killed," Getting said in a brief interview Wednesday with NBC station WOOD of Kalamazoo.

Arika Kruger, a witness, said she heard “a pop and a really big crash noise." Ambulances quickly arrived at the scene, she said, and "we didn't see anybody walk away."

"I pray for these families, I really do, because this is really tragic," Kruger told WOOD.

Organizers of the Ride of Silence, scheduled for 6 p.m. ET, said proceeds from T-shirt sales would be donated to the victims and their families through Kalamazoo Strong, a civic group formed after a gunman firing at random killed six people Feb. 20 in Kalamazoo.

"It's sad we haven't healed from the first mass tragedy that we experienced, and here we are hit with another one," said Rachel Kaiser, Kalamazoo Strong's business manager. "But Kalamazoo is strong, and we rally together. We take care of our own."

A "ghost bicycle" painted white was placed at the scene of the crash Wednesday as a memorial.

Seven hundred to 800 bicyclists are killed in crashes with motor vehicles every year across the United States, according to data compiled in November by the National Highway Traffic Safety Administration. Full data for 2015 haven't been reported, but the NHTSA said the number of deaths appeared to be trending higher.

U.S. bicyclist deaths by age and gender.

Bicycling activist groups called for quick action both to bring the driver to justice and to make the roads safer for cyclists.

Alex Doty, executive director of the League of American Bicyclists, based in Washington, D.C., described the encounter Wednesday as "one of the more horrific crashes ever seen for bicyclists."

"It is an extreme example showing how we must take stronger steps to make our roads safer for everyone," Doty said.

Renee Mitchell, president of the Kalamazoo Bicycle Club, said the group's thoughts were with the victims. She called the incident an "entirely preventable tragedy."

And John Lindemayer, executive director of the League of Michigan Bicyclists, based in Lansing, said in a statement: "The released details regarding the circumstances surrounding the incident suggest a willful and wanton disregard for human life. We urge local and state police to conduct a comprehensive investigation that will support swift prosecution to the fullest extent of the law."