The case involved an Anne Arundel County driver, a 50-year-old cyclist and a narrow two-lane road on a sunny afternoon.

A county grand jury heard the facts as presented by State’s Attorney Anne Colt Leitess and decided Friday that the driver should pay up to $2,000 in traffic fines for the Aug. 21 collision in which cyclist Trish Cunningham, 50, was struck from behind and killed.

“The grand jury determined that there was no probable cause to charge the driver with criminally negligent manslaughter,” Leitess said in a statement. When contacted by phone, she declined to discuss the case.

The decision sent a chill through the region’s vibrant cycling community.

“I’m scared to ride now,” said Mark Hamilton, a county resident who took up cycling this year while nursing a running injury. “Precedent has been set that if someone kills me, it will only cost them $2,000 in traffic fines.”

“Does this mean that when I get on my bike, my life is worth the same as a deer?” said Laura Manchester of Odenton.

Cycling advocate Alex Pline, who joined in street-corner protests and led a memorial ride after the accident, said the case to be made against driver Whitney DeCesaris was “clear-cut.”

“If there were ever a case that could clearly be made for [criminally negligent manslaughter], it’s this one,” said Alex Pline, who made a video that showed the approach to the accident scene as Cunningham and DeCesaris would have seen it.

“All you have to do is show a reasonable person that video, going up the hill, and they’d say, ‘No, no reasonable person would attempt to pass there,’ ” Pline said.

DeCesaris, 37, has not commented publicly on the accident. Calls and e-mails to her attorney’s office were not returned.

The competition for roadway space has accelerated in much of the world — London has had six cyclists killed this month — and the flash points have been particularly common in the United States, where a little more than a generation ago few adults used bikes for recreation or to commute. Drivers express their frustration with cyclists who fail to obey traffic laws or slow the passage of their vehicles.

Given the outcry from cyclists, runners and members of the large Catholic church where Cunningham attended Mass daily, Leitess had been under pressure to take action against DeCesaris.

DeCesaris married into a socially prominent family — the cancer building at the Anne Arundel Medical Center is named the Geaton and JoAnn DeCesaris Cancer Institute — and the family firm, Sonny DeCesaris and Sons, built many of the Maryland developments east of the District.

When Leitess took the case before a grand jury, some of those who had hoped for criminal charges thought she might be yielding to political pressure to stifle the case. But Jason A. Shapiro, former president of the Maryland Criminal Defense Attorneys’ Association, said that was the required procedure for a prosecutor considering felony charges.

“In this particular case, manslaughter was a possible charge,” Shapiro said. “The case cannot proceed to a circuit court on felony charges unless the grand jury decides there was probable cause.”

And the standard for probable cause for manslaughter, he said, is recklessness.

“Recklessness being a willful and wanton disregard for life or property,” he said. “It needs to be more than just a slight mistake — I understand that someone died, and that’s a horrible thing — but how many people make those slight mistakes every day? We don’t want be putting people in jail for making a mistake, but when people go the extra mile to be reckless, then they get prosecuted.”

Recklessness might be excessive speed, texting while driving or similar disregard for safety, he said.

“That’s up to the grand jury to decide,” Shapiro said.

The four traffic tickets issued to DeCesaris were for failing to exercise caution to avoid a collision, crossing the center line and unsafe passing, negligent driving, and failure to control speed.

The full police investigation has not been released.

The fatal crash occurred late on a sunny August afternoon. As Cunningham neared the crest of a hill, DeCesaris came upon her from behind on a narrow road without shoulders and attempted to pass.

A car suddenly appeared from the other direction, police were told, and DeCesaris swerved back to the right and slammed into Cunningham. Impact marks on the front bumper of DeCesaris’s minivan were about 10 inches from the right side of the vehicle, indicating that Cunningham was struck directly from behind, according to someone with knowledge of the investigation.

“Nothing can bring Trish back,” said family attorney Brent Carpenter, “but issuing a traffic ticket only compounds the tragedy and is unacceptable.”

A few hours after police delivered the traffic tickets to her door Friday, DeCesaris made her first public comment in a posting on Facebook.

“Starting today, I need to forget what’s gone, appreciate what still remains, and look forward to what’s coming next,” it said.