Watch: Latest News on Boston Marathon Bombings

Three people -- including an 8-year-old boy -- were killed and at least 176 injured Monday by twin explosions near the finish line of the Boston Marathon.

Dozens of Orange County residents were competing in the race, although many finished before the blasts, which rocked Copley Square shortly before 3 p.m. Eastern time. Calling the case a potential "terrorist" attack, FBI officials have taken the lead in the investigation.

A Mission Viejo runner who was less than a block away at the time described a dramatic scene of ambulances, police dogs and men in fatigues descending upon the square in black SUVs.

At first "I thought it was a gunshot," said Sally Boles, who was winding down from the race in a 13th-floor hotel room at the Boston Marriott Copley Place.

Boles didn't realize the noise was an explosion until about five minutes later, when a friend called from Southern California.

"It's crazy," she later told Patch. "There are 20 ambulances outside our hotel room. There are policemen everywhere. ... It's kind of like 9/11 all over again."

No suspects have been identified. Although a "person of interest" was interviewed by authorities Monday, an official later said the man was "not involved," according to the Boston Globe. 

Local Law Enforcement Reacts

As word of the deadly blasts spread, authorities across the nation went on alert.

In Orange County, Sheriff Sandra Hutchens promised "an increased presence at local sporting/entertainment venues and other large gathering places.''

Sheriff's Capt. Tim Moy, who is assigned to the department's Homeland Security division, said he was confident the incident was isolated based on information received from East Coast law enforcement officials.

Still, deputies will maintain "high visibility" to calm any fears, Moy said.

Orange County has a number of noteworthy entertainment venues, such as Disneyland, Angel Stadium and the Honda Center, so authorities are always on guard, Moy said.

"Anaheim does an outstanding job with that," Moy added. "And we work with Anaheim very closely."

Anyone going to an Anaheim Ducks or Angels game "should expect to see someone in blue or green nearby," Moy said. "We want to be proactive, not reactive."

Airport Security Tightened

Authorities were also on alert at John Wayne Airport, LAX and other local airports, where travelers should expect to see stepped-up security and longer lines, officials said.

"As a result of the incidents in Boston earlier today, the Airport is working closely with its security and law enforcement partners to ensure a safe, secure environment for travelers," said John Wayne spokeswoman Jenny Wedge.

Likewise, LAX and Ontario International promised to deploy "bomb-detection canines, additional patrol officers and security personnel,'' according to a statement from Los Angeles World Airports.

The Fate of Orange County Runners

Click on the links below for the lastest on marathon runners from Orange County Patch cities:

• Aliso Viejo
• Fountain Valley
• Laguna Beach
• Laguna Niguel-Dana Point
• Lake Forest
• Los Alamitos-Seal Beach
• Mission Viejo
• Newport Beach
• Rancho Santa Margarita
• San Clemente
• San Juan Capistrano

Boston Marathon People Finder

Google created a Boston Marathon people finder to help people locate friends or loved ones at the event.

Clarification

An earlier version of this story quoted a Wall Street Journal report that five additional undetonated explosives had been found in Boston. A law-enforcement official has since said closer examination of the devices led officials to doubt they were bombs.

Additional Stories on Patch

• Witness: 'All of a Sudden Everyone Turned Around and Was Running in the Opposite Direction'

- Written by Roy Rivenburg and Paige Austin with reporting from Peter Schelden, Nisha Gutierrez and City News Service.