'Zero' Arrests, Secured Obama Inauguration Rolls On Despite investigation into possible threat, events stayed largely under control.

Jan. 20, 2009 -- As Obama has been President Obama watches the inaugural parade from the White House reviewing stand and prepares to attend numerous inaugural balls, law enforcement officials confirm that the the crowds that turned out for the day's festivities remained in check.

"Zero. There have been no inaugural-related arrest[s] reported by any of our law enforcement partners today," the Secret Service said, as of 5 p.m. ET.

With the crowds dissipating now that Obama has been sworn in, law enforcement officials are breathing a sigh of relief that the headline event of the day passed without major incident.

A government official gave an unofficial estimate that the crowd topped 1.8 million, with 1.4 million on the Malland up to 400,000 in nearby streets.

The government doesn't give official crowd estimates, but it uses internal approximations to help dispatch resources.

Law enforcement officials had closed down public access to the parade route around 12:30 p.m. ET because it had begun to near its estimated capacity of up to 350,000.

And it was the massive crowd that presented a huge variable for law enforcement, though law enforcement sources said they do not have any information that there is an imminent threat to the inauguration festivities.

But out of caution, the Department of Homeland Security and the FBI issued a bulletin last night, expressing concern about "the threat individuals affiliated with al-Shabaab -- a radical Islamic extremist group active in Somalia -- may pose to the homeland, including locations and events of political significance, such as the upcoming presidential Inauguration."

The information, however, is "of limited specificity and uncertain credibility," the spokesman said, but "authorities at all levels are vigorously pursuing any lead relating to this threat information."

The DHS statement pointed out that large gatherings make for an attractive target for those looking to cause disruptions, but that officials "encourage the public attending inauguration events to go about their normal plans."

Noting that there is an "unprecedented level of security" for the inauguration, a DHS spokesman said that numerous federal law enforcement authorities are investigating and analyzing "recently received information about a potential threat on Inauguration Day."

ABC News recently reported that officials suspect that a group of men of Somali descent, who lived in the Minneapolis area, returned to Somalia for terror training. Officials have confirmed to ABC News that some of the individuals have returned to the United States.

The FBI and DHS bulletin notes that the men might have been associated with Shirwa Ahmed, a naturalized U.S. citizen who is believed to have blown himself up in a suicide attack in Somalia last October.

The bulletin states that officials released the information in an attempt to raise awareness and possibly obtain more information.

It also notes that "a threat to the inauguration does not necessarily mean that the target would be the Washington D.C., area, but could be elsewhere within the United States," but that past attacks attributed to the group have "focused on targets in Somalia."

But it continues, adding that "the organization possesses an international support network it may be able to leverage for international operations."

Huge Inauguration Crowds, Unprecedented Security

Inauguration activities are being protected by an army of police and military in one of the largest-scale security operations in U.S. history.

Heavily armed U.S. Coast Guard boats patrolling the rivers. SWAT teams at the ready. Military jets canvassing the restricted airspace surrounding the nation's capital.

A brand-new, fortified presidential limo ready for Obama.

Just after 9 a.m., the U.S. Park Police closed the entrances to a packed National Mall, directing the public instead to the Washington Monument grounds. The monument is just more than a mile away from the U.S. Capitol, where the swearing in took place.

The Metro subway system broke its ridership record, with trains shuttling 873,000 passengers to and from events by early evening.

Around 9:25 a.m., a 68-year-old woman at the Gallery Place-Chinatown Metro station fell onto the tracks, and was taken to a Washington Hospital Center with non-life threatening injuries to her arm. The station reopened after police investigated the scene.

D.C. and U.S. health officials said that as of early evening, there had been reports of more than 670 patients from the Mall and inauguration event areas, and that paramedics transported 44 to the hospital. The injured suffered mostly minor injuries from falls, hypothermia, and symptoms of cardiac stress.

Stepped-Up Security

Security for this inauguration promises to be the most expensive in the nation's history. And make no mistake, federal officials are deadly serious.

More than 8,000 police officers and an estimated 11,000 military personnel -- in addition to thousands of federal agents and undercover teams -- are looking for any hint of trouble in the huge crowd.

Inauguration Security: 'No Room for Mistakes'

Joseph Perssichini Jr., the assistant director in charge of the FBI's Washington field office, said he's conveyed the importance of the day's events to his agents.

"I just met with all the intelligence teams that'll be out on the street being our eyes and ears. and I told them that the American people are depending on you, and that's what we look at every day," he told ABC News before Inauguration Day.

"We cannot let our guard down, we have to give it 100 percent. There is no room for mistakes, and that's the motto we look at," he continued, noting that the agents understand that many variables could affect the security situation.

U.S. Secret Service Director Mark Sullivan said no resource would be spared.

"Whether it has to do with human resource asset[s], technology asset[s] -- there's no asset that we're not going to use to make this a safe event."

Sullivan added that the security preparations have been "very, very collaborative," a massive undertaking that involves "58 different federal, state and local law enforcement agencies," military and public safety entities.

"Everyone [is] working together to make this an event, an event that is not about security but an event that is about the president and our country," he added.

"It is going to be an interesting time here and no one is sparing any expense or holding back any capabilities that they have," Col. Dave Miles, the Air Force liaison to the Department of Homeland Security for the National Capital Region, said.

"Everyone understands you do not want to be second-guessed. ... But the measures in place, he said, should allow them to address "any contingency that's going to arise."

ABC News' Michelle Schlief contributed to this report.