Amtrak Special Operations Police patrol Union Station on Monday, days after the attacks in Paris. (Sarah L. Voisin/The Washington Post)

An alert is issued after four dark-skinned men with beards arouse suspicion at Metro’s Pentagon station. Two flights are diverted for bomb threats and a third is halted after passengers engage in “suspicious activity.” Motorists see a package fall from a truck and a major D.C. bridge is shut down as the “suspicious package” is investigated. A Belgian soccer star from Indonesia is mistaken for a terrorist.

In the days since the terrorist attacks in Paris, threats — real or imagined — have multiplied almost everywhere. Law enforcement officials have tightened security for the nation’s trains, subways and airports. And Americans are scared.

“I’ve overheard people talking about what happened in Paris,” said business traveler Kevin Boudreaux, 44, as television screens in the main hall of Reagan National Airport showed the latest updates from Paris.

“And I see people watching TV with worried expressions,” the software executive from Dallas said. “They’re shocked about what has happened.”

But such periods of heightened vigilance have also led to instances in which jittery travelers or security officials have singled out Muslims and others for special scrutiny.

In five attacks from 2012 to 2015, alleged assailants were born in Europe and ideologically​ motivated. (The Washington Post)

When security cameras captured images of four dark-skinned men with beards walking through the Pentagon Metro station, Transit Police issued an internal lookout for them. It was shared only with other law enforcement, but a television reporter got ahold of the bulletin and tweeted it — complete with a photo of the men. “This is scary,” said the tweet, which was shared thousands of times Wednesday and Thursday — until law enforcement said, in effect, never mind.

“It’s all fine,” Metro Transit Police Chief Ronald A. Pavlik Jr. said.

Each of the unidentified men was interviewed and deemed to have no “nexus to criminal activity,” Pavlik said. He declined to specify why the men had been sought except to say that their “travel habits” and “mannerisms” aroused suspicion.

So it goes in the age of terrorism, when one person’s reasonable suspicion can be another’s paranoia or, worse, outright prejudice.

FBI Director James B. Comey has said that officials are “not aware of any credible threat here of a Paris-type attack.” Yet as Americans hit the road for Thanksgiving, traditionally the busiest holiday travel week of the year, law enforcement and transportation officials face the challenges of keeping people moving and identifying potential threats while reassuring the public that no one will be subjected to racial or religious profiling.

The officials enjoy broad discretion about when to intervene. Pilots, who have ultimate control over their flights, can make on-the-spot decisions about whether to abort a flight. It means balancing the notion that it’s better to be safe than sorry with the principle that people should not be singled out based on stereotypes.

On Tuesday, two Paris-bound Air France jetliners — Flight 65 from Los Angeles, and Flight 55 from Washington Dulles International Airport in Virginia — were diverted because of anonymous bomb threats. Both were allowed to resume their trips after the threats proved to be unsubstantiated.

New York Police Department officers guard an entrance of the subway at Times Square in New York on Thursday. (Jewel Samad/AFP/Getty Images)

Also that day, four people were removed from a Chicago-bound jetliner at Baltimore-Washington International Marshall Airport.

The Chicago Tribune, quoting passengers after the plane landed, said at least two of the people removed from the Spirit Airlines flight appeared to be of Middle Eastern descent and that one was watching a news report on his mobile phone as the plane taxied toward the runway.

A spokesman for the airline said the person was removed because he refused to heed safety protocols against using a cellphone during takeoff, not because of his race or ethnicity.

“He was asked to stop the behavior and did not comply,” said Stephen Schuler, a spokesman for Spirit. “We do not tolerate discrimination or remove passengers because of where they are from, their ethnicity, or their religion.”

Douglass Kidd, executive director of the National Association of Airline Passengers, said pilots rightly have the authority to halt or divert a flight when passengers violate Federal Aviation Administration regulations or pose some other risk. That includes a passenger refusing to shut off a smartphone or failing to remain seated on takeoff. But Kidd said his group worries less about fellow travelers than about airport employees who have access to planes and who he said are not always screened adequately.

“To us, it seems rather pointless to X-ray passengers’ checked bags, and then turn those bags over to airport workers who have not been screened,” Kidd said in an e-mail. “Who watches the watchman?”

Anxieties are especially high as an estimated 47 million Americans prepare to travel for Thanksgiving. Nearly 42 million people are expected to go by car, 3.6 million by air and 1.4 million by other means, including train and bus, according to AAA Travel.

The Council on American-Islamic Relations, a civil rights group for Muslims, said there is often a surge in anti-Muslim incidents after terrorist attacks involving Islamist extremists. Since heavily armed militants killed at least 129 people in Paris on Nov. 13, vandals have destroyed a Koran at a Texas mosque, and a mosque in Ontario, Canada, has been set afire. CAIR said racial and religious profiling on planes and other public transportation also increases after major terrorist attacks.

“Unfortunately, after incidents like the Paris terror attacks, we see these kinds of spikes in reports of flying while Muslim, or really, perceived to be Muslim,” said CAIR spokesman Ibrahim Hooper. “We’ve had Sikhs singled out because they wear a turban or a beard. We’ve had Hispanics singled out.”

Hooper said he understands that people are afraid but that their fear shouldn’t be projected onto innocent fliers.

“All it takes is ‘discomfort’ on the part of either a passenger or a crew member,” Hooper said.

Meanwhile, transportation officials have tightened security. The Washington Metropolitan Transit Authority expanded random screenings for explosives at station entrances and added officers across the Metro system. Amtrak added patrols, including additional K9 teams, and New York’s subway system beefed up security.

Amtrak Police Chief Polly Hanson said her officers are well trained and sophisticated enough to know the difference between discriminating against someone and perceiving a possible threat. She said most commuters were savvy, too.

“We really focus on the behavior and not the color of someone’s skin,” Hanson said in an interview.

Pavlik, citing top-level meetings with the FBI and other federal law enforcement officials, told Metro board directors Thursday that “there is no credible threat” against the system, but he assured the board that his force remains vigilant.

In an interview, Pavlik said the number of people reporting suspicious activity since the Paris attack has surged fivefold. And he said it was something officials welcomed.

He noted that since the Paris attacks, the agency has seen about a 500 percent increase in “our abandoned items/suspicious package reporting.”

“We want to continue to see that reporting,” Pavlik said. “The tips, the calls — don’t stop. Don’t become complacent.”

Nisa Muhammad, who is in Hartford Seminary’s Islamic chaplaincy program, arrived at Washington’s Union Station on Wednesday after an all-night bus ride from Connecticut. Law enforcement was out in force at the bustling station, with some officers patrolling in pairs and carrying military-style semiautomatic rifles.

Muhammad, wearing a headscarf, said she has not been treated any differently than other passengers since the Paris attacks. But she has friends who have been singled out, perhaps because they are of Middle Eastern descent. Muhammad is African American, born in the United States.

She said that many people lose sight that millions of Muslims around the world observe their faith peacefully.

“Islam is not the problem. There are over a billion Muslims in the world, and a very minuscule number of misfits are creating problems,” she said.

Sheldon Thomas of Cincinnati said friends were more worried about his travels than he was. He said he didn’t think twice about making the trip to the D.C. area for Thanksgiving.

But he did admit to feeling a little uneasy after watching a segment on television in which testers were able to penetrate airport security with banned objects in three out of four instances.

“Now that gave me pause,” said Thomas, 49. “But I think it’s important that we just keep living.”

Correction: An earlier version of this story mistakenly reported that security concerns were raised at the Pentagon City Metro station. It was at the Pentagon station.

Paul Duggan, Dana Hedgpeth, Faiz Siddiqui and Ellen Nakashima contributed to this report.