More security officers than usual patrolled downtown Washington on Saturday as government officials continued investigating a possible terrorist attack timed to the anniversary of Sept. 11.

But there have been no arrests, and investigators said it is unclear whether the threat is real. They said little had changed since Friday, when the information about a possible attack on Washington or New York was characterized as raw intelligence that had not been corroborated elsewhere.

A federal law enforcement official said that as of Saturday, investigators had uncovered “no evidence to substantiate” the threat.

“This is what we do every day of the year in the absence of the public’s attention,” said another federal law enforcement official, who spoke on the condition of anonymity because the investigation is not public. “We receive threat information — varying in credibility and specificity — and run it out. Often, it results in no arrests.”

Security was tight in Washington and New York. Late Saturday afternoon, some of the gates in Concourse B at Dulles International Airport were shut down after what was thought to be a suspicious item was discovered in cargo, according to an airport spokesman. The gates were reopened after a more thorough check determined that the cargo pallet was harmless, the spokesman said.

In New York, police set up vehicle checkpoints at bridges and tunnels and increased their presence at the Grand Central and Times Square subway stations. Many of these plans had been in place for months, though, well before there was any word of a potential terrorist plot.

Security was stepped up in front of the White House as uniformed officers blocked off Pennsylvania Avenue, prohibiting tourists from crossing the sidewalk and gathering near the iron fence.

“We need you to stay on the sidewalk, please,” an officer yelled as a group of tourists spilled into the street while taking a picture.

On the Metro system, some riders said they were cautious, seeing extra personnel around as trains single-tracked because of regularly scheduled maintenance work and because of the Sept. 11 anniversary. There have been no specific threats reported to the rail system.

Other riders said they were not fazed by possible threats and had no plans to change their travel routine.

“I am kind of worried, but I don’t want to be paranoid,” said Stephanie Hauke of Germany, as she waited for a train at Union Station. “You don’t want to underestimate a threat.” She said she was debating whether to alter her plans to take the Metro on Sunday to Eastern Market because it is near the U.S. Capitol.

Because anything in the current climate could be significant or nothing, the Prince George’s County Police Department notified federal agencies and neighboring jurisdictions about two U-Haul vans that were stolen in the county Friday night.

“We are not viewing this as being terror-related — we are viewing this as two stolen vans — but in the current 9/11 climate, we are taking extra precautions,” said Evan Baxter, a spokesman for the department.

Later Saturday, Prince George’s police said that one of the vans was found empty in the 900 block of Marcy Avenue in the Glassmanor area. They were searching for the driver.

Still, the reports of a possible terrorist threat didn’t seem to jangle the nerves of people in the area.

With sun shining and a slight breeze Saturday afternoon, people wandered the Capitol grounds and snapped pictures as a helicopter flew overhead.

“I don’t feel nervous,” said Keith Dykstra, as he headed to a grassy area near the Capitol to read a book on India. “I don’t want to live my life in fear.”

Jim Schmaedeka of Clara City, Minn., said he felt as though security in the city was good. He and his wife were visiting with his daughter and son-in-law who had come up from Richmond.

“I am thinking this is one of the safest places to be,” he said.

Staff writers Jerry Markon and Martin Weil contributed to this report.