BOSTON (Reuters) - Even as the police prepare for a massive presence at Boston's first marathon since the 2013 bombing attack that killed three people and injured 264, officials acknowledge the sheer scale of the event poses inevitable security risks.
More than 3,500 officers will be stationed along the 42.2km course starting in suburban Hopkinton, Massachusetts, and ending among throngs of spectators at the bars and restaurants of Boylston Street, where two homemade pressure-cooker bombs ripped through the crowd last year.
While officials said they are not aware of any specific threat to the 118th Boston Marathon, one of the world's most prestigious races, they face a challenge of increasing security without taking a stance that is so aggressive it drives spectators away, security experts said.
"The police have to walk a delicate line between trying to convey to people that they are safe and not sending the message that, because of the militarised presence, they have something to fear," said Mr Tom Nolan, a former Boston Police Department official who now serves as chairman of the criminal justice department at the State University of New York at Plattsburgh.
"The public may not necessarily be reassured about their safety when they see police officers with machine guns and military uniforms and dogs," Mr Nolan said.
The race, on April 21 this year, is held on the state holiday of Patriots' Day, when schools as well as some businesses are closed, and tens of thousands of spectators typically clog the finish line area. It was there that ethnic Chechen brothers Dzhokhar and Tamerlan Tsarnaev placed the bombs they carried in black backpacks last year.
Knapsacks and large bags will be banned at the finish line and police are warning fans to expect checkpoints and inspections as they approach the course.
"In this world, you never eliminate risk. You never bring it down to zero. But we are working very hard at reducing the risk," State Police Colonel Timothy Alben told reporters.
Runners also face new restrictions, including a ban on bags at the starting line. That is a concern for athletes who in years past counted on bringing clothing to keep themselves warm while waiting for the race to start.
Weather conditions in Boston are highly variable in mid-April: While last year's race saw near-ideal temperatures of 18 deg C, runners in 2007 waited in wind and driving rain for the chilly 8 deg C start.
With 36,000 participants registered this year - the second-largest field in Boston Marathon history, with 9,000 more runners than last year - it means that the last of the race's four waves of competitors will depart Hopkinton two hours and 45 minutes after the starter's gun is fired at 8.50am.
The restrictions have irked some veteran racers.
"The people who caused the problem were not the runners. So security precautions that affect the runners and make it harder for us to run a marathon are not really addressing the problem," said Mr Ray Charbonneau, 52, of Arlington, Massachusetts.
Last year, he crossed the finish line about seven minutes before the bombs exploded, close enough to see and hear the blasts without suffering injury. Despite his misgivings, he plans to run his sixth Boston Marathon next month.
"I never had the slightest doubt that I wanted to go back," said Mr Charbonneau, who edited a book on last year's race called The 27th Mile. Race officials have also warned that so-called "bandit" runners, the unregistered athletes who were tolerated as they ran all or part of the face in past years, will be pulled off the course this year.
While heightened security along the course and in Boston on race day will be the obvious change, local and national law enforcement agencies will also be running down security-related leads ahead of the race, said Mr Joseph Wippl, a former Central Intelligence Agency official who now serves as a professor of international relations at Boston University.
"Any intelligence on anybody in the area and I'm sure the FBI or local law enforcement will pay them a visit," Mr Wippl said.
Even with additional security, some runners admit to safety concerns.
Mr Andrew Duffy will be among a group of Boston University runners honoring Chinese exchange student Lu Lingzi, one of the three people killed by the blasts.
Mr Duffy, who met his wife around the time he ran his first Boston Marathon in 1995, said strong personal memories brought him back but he acknowledged he has asked his family to stay away from the finish line on race day.
"I talked to my wife about that just today," Mr Duffy said. "We have three kids and everything, and I'm a little hesitant about having them around the finish area."