Cannons began to boom at 6 a.m., followed by the "rat-a-tat" of musket fire.
Even after daybreak, people in the crowd estimated at 10,000 couldn't see more than 50 feet in front of them.
They could hear the gunfire moving back and forth across the cornfield, visualizing in their minds Union Gen. Joseph Hooker's corps attacking Gen. Thomas "Stonewall" Jackson's wing in the Cornfield and the East Woods.
They could also smell the heavy mix of gunpowder and campfire smoke.
About 7 a.m., the fog lifted like a stage curtain and revealed the awesome scene of the decimated cornfield and an estimated 12,000 re-enactors.
Then came the climax of the battle. Confederate Gen. John Bell Hood charged his Texas Brigade into the cornfield, where more than 80 percent were killed during the real battle.
Hood was portrayed by Patrick Gorman, the same actor who played him in the movie "Gettysburg."
The real battle lasted four hours, but the re-enactment was over after about 90 minutes.
It ended the same way, essentially a standoff after both sides were too tired to fight any more, Frye said.
"It felt like the real thing," said spectator Joe Pillis, 42, of Hagerstown.
Before dawn, Doug Dobbs, 43, of Hagerstown, thought he spotted a Confederate line moving in the shadows.
"They talk about the confusion of the battle. You're now experiencing it," said Dobbs, a re-enactor who was sidelined by stomach problems.
The fog made the battle more real for re-enactors.
"We could not have scripted this any better," Frye said.
Confederate re-enactor Mike Hendricks, 36, of Virginia Beach, Va., said he couldn't see his hand in front of his face at times.
At times, tempers flared among the re-enactors, said Douglas Ridenour, 46, of Hagerstown.
"It was very chaotic, just like it would have been," Ridenour said.
There was no fog that afternoon, when the Sunken Road - or Bloody Lane - battle scenario was re-enacted before an even larger crowd.
Still, many spectators had problems getting a full picture of the battlefield action, which started on a large field north of Rench Road and ended in a smaller field to the south.
Pat O'Malley, of Columbia, Md., said she and her family came early and set up on a small hill overlooking both fields to get a complete view of the scenario.
"We had good seats until everybody ran in front and we lost out," said O'Malley, who said she stayed put anyway because her granddaughter had fallen asleep and she didn't want to wake her.
She said she and daughter Ellen Morucci were able to see the north field but lost sight when Union troops marched south toward Confederate troops entrenched behind the Sunken Road fence.
The heart of the 90-minute confrontation played out there, as Confederate soldiers fought to hold the line against a much larger force of Union soldiers.
"I didn't see a thing," O'Malley said.
Husband Ed O'Malley said he found the opposite problem after working his way up to the edge of the south battlefield with his 13-year-old grandson, Sean Morucci.
O'Malley said he couldn't see much of what was going on until the battle moved south.
Even then, he said, he was having problems with camera crews and photographers standing in his way.
What he did see impressed him, he said.
"It's hard to imagine. It's hard not to be moved," he said.
Watching the Sunken Road scenario was the highlight of the weekend for 14-year-old Jesse Henry, who said he was especially interested in that part of the battle because one of his relatives played an important role in it.
Henry said his great-great-granduncle Thomas Frances Meagher, led troops as a brigadier general of the Irish Brigade.
"They lost over half their men, and they led the charge," said Henry, who said he and his family came down from Long Beach, N.Y., for the Battle of Antietam commemoration event.
Wearing a Union cap bearing Irish Brigade insignia, Henry watched the battle closely with a pair of binoculars.
"I'm going to be doing this as soon as I'm old enough," he said.
The Sunken Road re-enactment was worth braving nearly two hours of traffic on Sharpsburg Pike and Rench Road, said Ed and Linda Welch, of Baltimore, who got to the battlefield about 15 minutes before the afternoon battle started.
"I'm a Civil War buff," Ed Welch said.
Dennis Bennett, of Berkeley Springs, W.Va. criticized organizers for inadequate signage and general confusion about where events were supposed to occur.
"It was just kind of chaos," Bennett said.