CHICAGO -- The concern is always somewhere in the back of athletes' minds. A whisper of a worry that a drunk or angry fan might cross the line.

Most days, however, the playing field serves as a sanctuary, and the worries are set aside.

But not Thursday night. On an otherwise tranquil evening, in an otherwise nondescript game, the fears came to horrifying and vivid life when Royals coach Tom Gamboa was attacked by a father and son who came out of the seats. A folded-up pocket knife was found on the ground afterward.

Suddenly, there is a whole new batch of questions about security at ballparks.

"You're not safe anywhere," Royals outfielder Carlos Beltran said. "We think we're safe at the ballpark. What happened, that tells us no matter where we are, we're not safe."

The Royals rushed to Gamboa's aid, and the 54-year-old first-base coach escaped with a few cuts and bruises.

The father, identified by police as William Ligue Jr. of suburban Alsip, Ill., was charged Friday with aggravated battery, a felony. He was in custody and was expected to make a court appearance Saturday.

Ligue's 15-year-old son was charged with two juvenile counts of aggravated battery, one for attacking Gamboa and the other for hitting a White Sox security guard, an off-duty police officer. He was released to his mother's custody Friday morning. Calls to her home went unanswered.

"Certainly, I believe they should be prosecuted, because if we just slap people on the wrist or allow the excuse that they were drunk or on drugs or something, that to me, in my opinion, opens the door for other people to do this," Gamboa said Friday in an interview with Sporting News Radio.

"There is no possible excuse for people at a sporting event to be on the field unauthorized," he added. "It is just ludicrous."

But it happens -- too often to be ignored.

Who hasn't seen fans racing around a field until they're tackled by a security guard? It seems like a harmless stunt, and most players don't even move when it happens.

Occasionally, though, it turns ugly, like when a 23-year-old fan attacked Houston right fielder Bill Spiers on Sept. 24, 1999, in Milwaukee. Spiers ended up with a welt under his left eye, a bloody nose and whiplash.

On Sept. 28, 1995, Cubs reliever Randy Myers was charged by a 27-year-old bond trader who ran out of the stands at Wrigley Field. Myers saw the man coming, dropped his glove and knocked him down with his forearm.

Or in the most horrific incident, a fan stabbed Monica Seles in the back during an April 1993 match in Hamburg, Germany.

"It is a fear of players," Royals outfielder Chuck Knoblauch said. "Because it seems like the fans continue to get more and more hostile."

It wasn't clear what provoked the attack. As they were being put into police cars, both the father and son contended there had been an exchange with Gamboa.

"He got what he deserved," Ligue said.

But Gamboa denied there was an exchange.

"I have never at any time ever verbally or physically acknowledged the people in the stands," Gamboa told the Sporting News Radio. "That did not happen in any way shape or form."

So how can incidents like this be prevented in the future? Or can they?

Citing security reasons, White Sox spokesman Scott Reifert said he couldn't discuss what changes the team might be making or whether security would be increased. Security guards -- all of whom are off-duty police officers -- already are positioned at both ends of each dugout and elsewhere on the field.

At Kauffman Stadium, where the Royals were to begin their final homestand Friday night, security chief Chris Richardson said he fears a copycat attack.

"You would be a fool not to be on your guard for that," Richardson said. "Not only here, but at any other stadium. As folks charged with maintaining a safe atmosphere for everybody, we should make sure we review where we are and make sure there are no potential holes in our security anywhere."

Security at ballparks tightened after the Sept. 11 terrorist attacks. Fans are restricted from bringing in large bags, and smaller ones are searched. But like the screenings at airports, they've relaxed with time.

And with no metal detectors at ballparks, there's nothing to prevent someone from bringing in a weapon.

"It doesn't matter how much you beef up security," Royals starter Paul Byrd said. "Anybody can get by at any minute."

Ligue and his son hopped a small barrier to get on the field, but there's no guarantee a larger wall would have kept them out.

Two years ago at Wrigley Field, a fan leaned over a chest-high wall to snatch the cap off Dodgers catcher Chad Kreuter's head. Kreuter gave chase into the stands, his teammates followed and soon the Dodgers and fans were mixing it up in the seats.

Short of putting up glass walls around playing fields and turning every park into an NHL arena, there's really no way to keep fans on their side of the game.

"Ultimately, these are two random idiots who acted in that way," Reifert said. "I don't think you ever lose that percentage chance of that happening.

"You can put up hurdle after hurdle after hurdle. But ultimately, people are responsible for their own actions."