NEW YORK -- Entering a big league ballpark will be a bit like going through an airport by 2015.

Major League Baseball has told its 30 teams they must implement security screening for fans by then, either with hand-held metal detection or walk-through magnetometers.

"This procedure, which results from MLB's continuing work with the Department of Homeland Security to standardize security practices across the game, will be in addition to bag checks that are now uniform throughout MLB," baseball spokesman Michael Teevan said Tuesday.

The Seattle Mariners announced Tuesday that fans entering Safeco Field will have to walk through metal detectors starting with this year's opener.

Boston, the New York Mets, Oakland, Pittsburgh and San Francisco were among the teams that experimented with screening at times last year. Security gained more intense focus after three people were killed and more than 260 wounded in when two bombs were set off at the Boston Marathon finish line.

"We conducted testing of these measures at the All-Star game and at both World Series venues last year, and we were pleased that it was effective and received without issue from fans," Teevan said.

MLB is being assisted by CEIA USA Ltd., which manufactures walk-through metal detectors, and the security company GSIS.

Big league teams have generally limited bags that fans bring to 16 inches by 16 inches by 8 inches.

In an effort to increase security this season, the NFL limited the size and type of bags fans can bring to games. With the exception of medically necessary items, only clear plastic, vinyl or PVC bags no larger than 12 inches by 6 inches by 12 inches have been allowed. One-gallon clear plastic freezer bags also were permitted along with clear plastic bags approximately the size of someone's hand.