Confronted by a distressing human toll in Boston and comforted by few answers from investigators, D.C.-area officials were on heightened alert Tuesday, even as they sought to reassure residents and visitors that they knew of no threats to the region.

"You're not going to see a lot of security presence because much of the security that we put in place isn't visible," D.C. Mayor Vincent Gray said after he led an Emancipation Day parade. "We recognize that as the nation's capital we always are potentially vulnerable."

Although there were few indications of tightened security along the parade route, which stretched nearly a mile along Pennsylvania Avenue and ended two blocks from the White House, two city officials said that authorities with bomb-sniffing dogs had conducted a massive security sweep ahead of the event.

The officials requested anonymity to discuss law enforcement tactics, but police dogs were visible to onlookers.

What to watch for
Federal and local officials on Tuesday were urging people to report suspicious activity to law enforcement. Authorities suggested dialing 911 to assist police with their effort to conduct surveillance on key facilities or to provide sensitive information about those locations. They also asked for reports of suspicious people, attempts to breach security and unusual efforts to obtain supplies that could be used in an attack.

Jim Hollis, who was visiting D.C. from a Boston suburb, said he didn't hesitate to proceed with his plans to tour the city's sights.

"We were going at it," Hollis said near the National Archives. "If you live in fear, you're letting them win."

Hollis said he hadn't noticed increases in security.

"I haven't really seen a change," he said. "If I was back in Boston, I'd probably see a bigger change."

At Freedom Plaza, Ward 7 resident Juanita Fairchild said she was not reluctant to attend the parade that coincided with her birthday.

"I have a firm belief that I'm not going until my time," Fairchild said. "God's not going to let anything happen to us."

But law enforcement officials were taking no chances.

Authorities removed Capitol Hill trash cans to prevent someone from planting explosives in them, and Verizon Center said it was increasing security.

Maryland and Virginia authorities said they were unaware of credible threats, but some agencies said they had increased patrols.

Metro also said more of the transit agency's police officers would be on duty Tuesday.

Staff Writer Liz Essley contributed to this report.