A grand jury has determined that Tony Stewart will not face charges for his role in the on-track death of sprint car driver Kevin Ward Jr. (0:39)

CANANDAIGUA, N.Y. -- Prosecutors announced Wednesday that a grand jury had decided against bringing criminal charges against three-time NASCAR champion Tony Stewart in the August death of a driver at a sprint car race in upstate New York.

Ontario County District Attorney Michael Tantillo said the victim, Kevin Ward Jr., was under the influence of marijuana the night of the accident "enough to impair judgment." Tantillo also said two videos examined by investigators showed "no aberrational driving by Tony Stewart."

The decision came almost seven weeks after Stewart's car struck and killed Ward during a dirt track race Aug. 9.

Stewart, the brash and popular NASCAR driver known as "Smoke," spent three weeks in seclusion following what he called a tragic accident before quietly returning to the Sprint Cup circuit. One of the biggest stars in the garage, Stewart has 48 Cup wins in 546 career starts but is winless this year and did not make the championship Chase field.

"This has been the toughest and most emotional experience of my life, and it will stay with me forever," Stewart said in a prepared statement. "While much of the attention has been on me, it's important to remember a young man lost his life. Kevin Ward Jr.'s family and friends will always be in my thoughts and prayers."

Tantillo could have determined the case on his own but last week said he would take it to a grand jury. Experts said it would have been difficult to prove criminal intent, but the prosecutor asked the grand jury to consider charges of second-degree manslaughter and criminally negligent homicide.

"There were not 12 votes to either charge," he said.

"There are no winners in tragedy. Our thoughts and prayers remain with the Kevin Ward Jr. family and Tony Stewart as they all cope with this tragic incident at Canandaigua Motorsports Park," NASCAR said in a statement. "This has been a difficult time for everyone involved and we have respected the local authorities responsible for reviewing this case."

Sheriff Philip Povero spent weeks investigating, several times saying investigators did not have evidence to suggest Stewart meant to harm the other driver at the track in tiny Canandaigua. Ward had spun while racing alongside Stewart, and then the 20-year-old climbed out of his car and walked down the track, waving his arms in an apparent attempt to confront the 43-year-old NASCAR veteran.

Authorities said the first car to pass Ward had to swerve to avoid hitting him. The front of Stewart's car appeared to clear Ward, but Ward was struck by the right rear tire and hurtled through the air. He died of blunt force trauma. His father later told a newspaper there was "no reason" for the death given Stewart's skills and experience.

Ward's family, in a statement to ESPN on Wednesday, said: "Our son got out of his car during caution, while the race was suspended. All other vehicles were reducing speed and not accelerating except for Tony Stewart, who intentionally tried to intimidate Kevin by accelerating and sliding his car towards him, causing this tragedy.

"The focus should be on the actions of Tony Stewart and not Kevin. The matter is not at rest and we will pursue all remedies, in fairness to Kevin."

Brad Keselowski, who leads the Chase for the Cup standings after two races, weighed in on the results of the investigation on Twitter.

Can't believe what I'm reading about Tony Stewart's case. Why didn't they release this sooner?!?! - Brad Keselowski (@keselowski) September 24, 2014

The sheriff asked in the days after Ward's death for spectators to turn over photos and videos of the crash as investigators worked to reconstruct the accident. Among the things being looked at were the dim lighting, how muddy it was and whether Ward's dark firesuit played a role in his death, given the conditions.

A fan's video was widely circulated after Ward's death, but details of the second video obtained by the sheriff have not been made public.

Stewart, who Povero described as "visibly shaken" after Ward's death, vowed to cooperate with the investigation but did not testify before the grand jury. He issued a brief statement expressing deep sadness and then dropped off the radar, missing races at Watkins Glen, Michigan and Bristol before coming back for the Aug. 31 race at Atlanta.

Looking far different from the fiery driver his fans love, Stewart quietly read a statement at a news conference in which he said the death of a driver he hit was "one of the toughest tragedies I've ever had to deal with."

Timeline of Stewart-Ward Incident Aug. 9: Tony Stewart's car strikes and kills Kevin Ward Jr. after Ward walked toward Stewart's car under caution at Canandaigua Motorsports Park in New York. Ward's car hit the wall while racing closely with Stewart.

Aug. 10: Stewart withdraws from Sprint Cup Series race at Watkins Glen; Ontario County (N.Y.) Sheriff Philip Povero states there are no criminal charges pending.

Aug. 11: Autopsy reveals Ward died of blunt force trauma; Stewart withdraws from a dirt track race in Indiana set for Aug. 16; Povero reiterates there are no criminal charges pending. Aug. 12: Povero says the investigation could last "two weeks or more"; Kevin Ward Sr. has harsh words for Stewart.

Aug. 14: Services held for Kevin Ward Jr. in New York; Stewart withdraws from Michigan; Jeff Burton named replacement.

Aug. 15: NASCAR announces rule change to keep drivers in their cars after accidents.

Aug. 21: Stewart withdraws from Bristol; Burton named replacement.

Aug. 28: Team announces Stewart will return at Atlanta.

Aug. 29: Stewart reads a prepared statement, doesn't take questions from media; NASCAR president Mike Helton says Stewart is eligible for the Chase if he qualifies.

Sept. 11: Povero announces investigation has concluded; findings are turned over to district attorney for review.

Sept. 16: Ontario County District Attorney Michael Tantillo announces evidence will be submitted to a grand jury.

Sept. 23: Grand jury hears testimony.

Sept. 24: Tantillo announces findings of grand jury, saying Stewart will not be charged.

"This is something that will definitely affect my life forever," Stewart said then. "This is a sadness and a pain I hope no one has to experience in their life. That being said, I know that the pain and mourning that Kevin Ward's family and friends are experiencing is something that I can't possibly imagine."

Stewart's peers were protective of him as questions emerged in the aftermath of the crash, and it pained them that Stewart was grieving in private and had cut off communication with so many of them. They welcomed him back in Atlanta, and fans gave him a robust cheer too. Two days later during the race, his No. 14 Chevrolet slammed into the wall twice and Stewart settled for a 41st-place finish.

After Ward's death, NASCAR announced a rule that prohibits drivers from climbing out of a crashed or disabled vehicle -- unless it is on fire -- until safety personnel arrive. The series also cleared the way for Stewart to make its Chase for the Sprint Cup championship with a win, despite missing the three races.

Stewart, who is from Columbus, Indiana, has long been one of the most proficient drivers in racing, winning in every kind of series, from sprint cars to the elite Sprint Cup Series. He has for years taken part in little races in nondescript towns because he loves the thrill of the high horsepower, lightweight cars skidding around the dirt.

He rarely made his schedule public, popping up when he pleased, and he was welcome at the clay track at Canandaigua Motorsports Park the night before the NASCAR race in nearby Watkins Glen.

There have been few criminal prosecutions stemming from incidents that occur during competition.

There were two cases in the past 14 years in which NHL players were charged for their actions on the ice. Marty McSorley was convicted of assault with a weapon for hitting Donald Brashear with a stick during a 2000 game, and Todd Bertuzzi pleaded guilty to assault causing bodily harm for punching Colorado's Steve Moore in 2004.

But it's extremely rare for incidents during auto races to bring police scrutiny, and Povero emphasized that this was an "on-track crash investigation."

Research by The Associated Press dating to 2003 turned up no cases in which a driver was charged for his role in an on-track incident. Team owner James Sink was found guilty of misdemeanor assault of driver Maynard Peters after a bloody 2005 postrace fight in North Carolina. In 2003, NASCAR driver Jimmy Spencer was under investigation for assault after punching Kurt Busch in the face following a race at Michigan, but Busch asked for his complaint to be dropped.

Legal experts believe the case against Stewart would have been difficult to prove.

"Death happens in a very dangerous sport, and this was driver vs. driver and Ward stepped on the track," said David Weinstein, a former state and federal prosecutor in Miami who is now in private practice. "The only person who really knows what happened is Tony Stewart, and it's impossible for a prosecutor to get inside his head."

Mobil 1, the primary sponsor for Stewart, said Wednesday that its funding for the driver remains unchanged. Bass Pro Shops and Rush Truck Centers, two of Stewart's other top sponsors, both strongly supported the driver when he made his return to racing.

Information from The Associated Press is included in this report.