MLB said it was looking at the Padres' handling of medical information of players they were trading. The league's investigators interviewed officials from both teams and then submitted their findings to commissioner Rob Manfred.
MLB, which announced the suspension Thursday, said it considers the matter closed.
Padres officials instructed their organization's athletic trainers to maintain two distinct files of medical information on their players: one for industry consumption and the other for internal use, multiple sources told ESPN.
Trainers were told in meetings during spring training that the distinction was meant to better position the team for trades, according to two sources with direct knowledge of what was said.
"I accept full responsibility for issues related to the oversight of our medical administration and record keeping," Preller said in a statement. "I want to emphasize that there was no malicious intent on the part of me, or anyone on my staff, to conceal information or disregard MLB's recommended guidelines. This has been a learning process for me. I will serve my punishment and look forward to being back on the job in 30 days."
On July 14, the Red Sox traded one of their best pitching prospects, Anderson Espinoza, for Pomeranz, San Diego's All-Star left-hander. Sources within the Boston organization say it wasn't until after the deal was made that they became aware of some of the measures the Padres had taken.
According to sources familiar with the fallout from some of the Padres' midseason deals, officials from at least three teams that made trades with San Diego -- the Red Sox, Miami Marlins and Chicago White Sox -- were enraged by what they perceived to be strategic deception: veiling medical information that could have been pivotal in trade discussions. At least one other team reached out to the commissioner's office with a complaint, sources told ESPN.
A deadline deal that sent pitcher Colin Rea to Miami was effectively reversed after Rea was unable to make it through his first start with the Marlins because of an elbow injury. After Rea was sent back to San Diego, Marlins president of baseball operations Mike Hill said that the teams exchanged medical records before the trade and there were no concerns.
Preller's suspension was specific to the Pomeranz trade but does not preclude MLB from pursuing further questions about the Padres' actions if information develops, a source said.
In a statement released Thursday, Padres executive chairman Ron Fowler, managing partner Peter Seidler and chief executive officer Mike Dee said the team will "fully comply with Commissioner Manfred's recommendations pertaining to changes with our medical administration and record keeping."
"Rest assured, we will leave no stone unturned in developing comprehensive processes to remediate this unintentional, but inexcusable, occurrence," they said in the statement. "To be clear, we believe that there was no intent on the part of A.J. Preller or other members of our baseball operations staff to mislead other clubs. We are obviously disappointed that we will lose A.J.'s services for 30 days, but will work closely with him upon his reinstatement to ensure that this unfortunate set of circumstances does not happen again."
Through a team spokesman, Red Sox president of baseball operations Dave Dombrowski said he's "not able to comment on an MLB investigation."
All MLB teams feed medical information into a central database known as the Sutton Medical System, designed to both maintain the privacy of individual players and to be accessible to teams when needed -- such as when trades are made.
Any time a player goes into the training room and receives treatment -- down to hot tubs, aspirin and anti-inflammatories -- those details are supposed to be entered into records.
When teams close in on trades, the athletic trainers usually exchange codes needed to access the medical information stored on the players in question so inquiring teams can learn about a player's physical condition.
Two sources with direct knowledge of the Padres' meetings told ESPN that the staffers were instructed by front-office officials to document medical details about players into the two separate systems. The athletic trainers were told to post the details of any disabled-list-related medical situations on MLB's central system, but they also were instructed to keep the specifics about preventive treatments only on the Padres' internal notes, sources said. One source defined the distinction in this way: If a player was treated for a sore hamstring or shoulder without being placed on the disabled list, that sort of information was to be kept in-house, for use within the organization only.
The athletic trainers were told that by splitting the medical files into two categories, the Padres would benefit in trade discussions, two sources with direct knowledge of those meetings said.
Preller previously worked for the Texas Rangers and, when he was overseeing their international operations in 2010, he was suspended for violations of baseball rules regarding signings.
He was hired by San Diego in August 2014 after years of developing a reputation for being one of the sport's best and most adept scouts. Preller has been lauded by the Padres' ownership for his work in stocking the team's farm system.
Shortly after Preller took over the Padres, the team was reprimanded by MLB for conducting a workout contrary to industry regulations.
Information from The Associated Press was used in this report.