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CHICAGO (AP) -- First base umpire Laz Diaz turned to watch a fly ball that was hit along the right-field line. A split-second later, a fan was grabbing him around the waist.

Just seven months after Kansas City coach Tom Gamboa was assaulted by a father and son who'd run on the same field, another fan at the Chicago White Sox's home park was shockingly attacking Diaz.

The fan entered the field not far from where Gamboa had been surprised and then pummeled in the first base coach's box last September.

"We spoke about it in the locker room, this is where Tom Gamboa got attacked. We thought nothing was going to happen, but it was a full moon out there," Diaz said.

"He just grabbed me by my waist and that's when I turned him around and got him down ... and everybody jumped on him."

Known as Comiskey Park when Gamboa was attacked, the facility is now called U.S. Cellular Field after a $68 million naming rights deal was reached before the season.

As the host for this July's All-Star Game, the stadium has already undergone a major sprucing up. Now it has a black eye, its safety record questioned by yet another ugly incident at a Royals-White Sox game.

"As a major league baseball player, you shouldn't have to worry about your health on the baseball field from the fans," Kansas City star Mike Sweeney said. "When they come on the field to do harm, that's when it gets scary."

Immediately after Carlos Lee flied out to end Chicago's eighth inning, a man ran on the field and tried to tackle Diaz.

Security and players rushed to the aid of the 40-year-old Diaz, who served in the U.S. Marine Corps Reserve. The fan was quickly taken away and Diaz, who was not hurt, finished the game.

"When I looked over, there was one of the fans from the stands and I just got him off me," Diaz said. "I just turned around and got him off me. The good hand-to-hand combat they taught me worked."

Police spokesman Wayne Frisbie said investigators wanted to talk with Diaz before deciding whether to file charges against the man. He remained in police custody Wednesday.

Frisbie said the man, who has not been identified, was treated for minor injuries at St. Anthony's Hospital in Chicago.

According to Chicago Police spokeswoman Officer Alice Casanova, police are leaning toward a charge of aggravated battery, a felony, but that must be approved by the prosecutor's office. The fan's name will not be released until he is formally charged.

Several Royals players could be seen kicking and stomping the fan while he was pinned down. When the attacker was put into a police car, his head was wrapped with a white bandage, soaked with blood near the right temple.

Royals right fielder Brandon Berger, who caught the ball for the final out of the inning, was one of the first to reach Diaz.

"You catch the ball, you look down and a guy's getting tackled and it's like, 'What's going on?' " Berger said.

Eerily, it was the first appearance by the Royals in Chicago since Gamboa, then Kansas City's first base coach, was attacked last Sept. 19

Now the Royals' bullpen coach, Gamboa said he thought security was tighter for his team's return. He felt safe -- at least before the game.

"Oh yeah, the assistant to major league security was at the game tonight. He told me before the game that they had beefed up the security, but the fans will always outnumber the security force, so there is only so much they can do," Gamboa said.

"I think people just have too much to drink," he added.

Kevin Hallinan, baseball's security head, was in San Francisco when he learned of the attack. He was on his way to Chicago to investigate.

"I'm going to be meeting with all concerned," he said. "I spoke with Diaz and he did say he thought security got out there in a hurry."

And the attack on Diaz wasn't the only incident involving rowdy spectators. It was just the most brazen.

Earlier, Tuesday night's game was delayed three times when fans ran onto the field before being tackled by security guards. All three were charged with trespassing, the White Sox said.

"The fans just seem to be getting on the field way too easy," Royals third baseman Joe Randa told ESPN Radio. "Guys are getting paranoid and are starting to look over their shoulders. One of the [fans] ran right by me ... I didn't know if he was coming after me or what.

"I don't know if it's the alcohol or if [fans] just want to be on TV. It's got to stop. Unless something gets done here, I can guarantee we're not going to step out on that field again."

Frisbie said all three were charged with misdemeanor criminal trespass to land. He identified them as Jeffrey Adams, 25; Albert Skutnik, 25; and Tom Skutnik, 20, all of Chicago.

"I don't know how it is when other teams are in town, but it seems like every time we are here, something crazy happens," Sweeney said. "Maybe they should bring more police in or put up some high fences so fans don't get on the field."

The White Sox said it was the actions of a few who don't represent their true fans.

"The behavior of four people attending tonight's game was reprehensible and will not be tolerated," the team said in a statement. "They will be prosecuted to the fullest extent the law allows."

The 55-year-old Gamboa still has minor hearing loss in his right ear from last year's attack.

The boy, 15 at the time, was sentenced to five years of probation and also ordered to undergo mandatory counseling and perform 30 hours of community service.

He was arrested along with his father, William Ligue Jr., who initially pleaded innocent to charges of aggravated battery and mob action. The case is still pending.

After the attack on Diaz, the Royals scored four runs in the top of the ninth for an 8-5 win -- their 11th victory in 12 games.

Sweeney hit a two-run homer to put the Royals ahead. Albie Lopez (2-0) won and Billy Koch (1-1) lost after blowing the save.

The benches emptied in the first when Miguel Asencio hit Chicago's Frank Thomas with a pitch for the second time this season. No punches were thrown.

Thomas homered in the eighth to give the White Sox a 5-4 lead.