The crowd was advised over public address systems and through social media to take cover Sunday afternoon when lightning and heavy rain hit the track near the end of the race.
But some NASCAR fans posted on the raceway's Facebook page that they never heard the weather-related announcements.
Pocono Raceway president and CEO Brandon Igdalsky expressed sorrow at a news conference Monday afternoon at the track, where a large U.S. flag flew at half-staff. He said that "fans are like family to us" and that he planned to visit other victims.
One bolt hit the grandstand parking area around 5 p.m. Sunday, killing Zimmerman and injuring eight others, Igdalsky said. A second possible strike came around 6:35 p.m., sending a ninth person to the hospital with minor injuries, he said.
Brian Mattson of Greentown, Pa., said he and friend Tom Deacher had just gotten into their truck to leave the track when they saw the first bolt hit about two car rows in front of them. Mattson said sparks flew "like a Roman candle" after the lightning hit a tailgating canopy next to a car.
"When the tent collapsed, I knew it wasn't right," said Deacher, of Mayfield, Pa.
They ran over and found two men on the ground. Deacher said he and others tried to administer CPR to the men until paramedics arrived. They don't know if one of the men was Zimmerman.
Communicating incoming weather is often a challenge for officials at tracks throughout the country. Most such facilities especially the 2.5-mile Pocono Raceway are massive, with fans spread among grandstand seating and a spacious infield where fans camp and tailgate.
NASCAR stays in contact with track officials when weather may affect a race, but it's the responsibility of track officials to communicate with race fans about advisories or severe storms approaching.
Decisions about proceeding with a race are typically made minute-by-minute, although there have been instances the last several years when NASCAR worked with track officials in advance of incoming weather in the interest of fan safety.
The decision to postpone a 2008 race at Richmond was made a day before the scheduled start because Tropical Storm Hanna was moving toward Virginia.
In 2010, all track activity at Talladega Superspeedway was canceled because of extended periods of severe weather. Track officials made the decision based on advice from the Talladega County Emergency Management Agency and the National Weather Service, which warned of potential tornadoes.
And this season, the Daytona 500 was postponed for the first time in its 54-year history.
That's one of the problems NASCAR has when rain does threaten an event: Fans feel cheated if they don't see a full race, and NASCAR's first priority is usually to try to wait out a storm in order to complete all the scheduled laps.
Ed Klima, director of emergency services at Dover International Speedway in Delaware, said that while "the facility is ultimately responsible for the fans' safety ... it's obviously very difficult to get people to leave if there's still cars going around the racetrack."
He also noted that racetracks are not built like NFL stadiums, which have concourses where fans can gather during inclement weather.
NASCAR's been known to wait hours to attempt to finish a race, and in 2009 they brought teams back the next day to finish the Coca-Cola 600 at Charlotte Motor Speedway.
There have been 19 lightning fatalities nationally so far this year, which is about average, according to John Jensenius, a lightning safety specialist with the National Weather Service.
"There is simply no safe place outside," said Jensenius.
The deaths have occurred while people were playing soccer, fishing, doing yard work, picking squash or berries, and simply at outdoor gatherings.
While not specifically addressing the strike at Pocono, Jensenius said that "typically in parking lots, we see lightning strike poles and then spreading out along the ground."