FRAMINGHAM, Mass. (AP) — The number of police officers patrolling this year’s Boston Marathon will be doubled to over 3,500, one year after two bombs exploded near the finish line, killing three people and injuring more than 260.

The enhanced police presence is part of a beefed-up security plan detailed Monday by public safety officials as they prepare for the April 21 marathon.

Spectators who plan to attend the marathon are being strongly discouraged from bringing backpacks, rolling bags, coolers and other large items, and are instead being asked to carry personal items in clear plastic bags. Anyone who does bring a bulky bag will be subject to search, officials said.

The new guidelines for spectators are similar to rules for runners made public several weeks ago by the Boston Athletic Association.

Authorities said they sought to strike a balance between keeping the traditional feel and character of the marathon and tightening security in response to last year’s deadly terror attack.

“We are confident that the overall experience of runners and spectators will not be impacted, and that all will enjoy a fun, festive and family-oriented day,” said Kurt Schwartz, director of the Massachusetts Emergency Management Agency.

Like runners, spectators are also being told they cannot bring containers with more than 1 liter of liquid and cannot wear bulky costumes or anything that covers their faces.

Unregistered runners known as “bandits” will be prohibited this year. Officials said the move was necessary because or security concerns and because organizers are expecting 9,000 additional runners this year — bringing the total field to 36,000 — and double the typical number of spectators for a crowd of up to one million.