Nearly four years after Alex Rodriguez admitted that he used steroids for a portion of his professional career, he continues to be a source of curiosity and concern for Major League Baseball and the Yankees.

Enlarge This Image Sean Proctor for The New York Times Baseball officials are investigating a trainer linked to Alex Rodriguez.

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Rodriguez turns 38 in July, appears increasingly brittle and no longer seems assured of surpassing Barry Bonds as baseball’s career home run leader. He is also just halfway through a 10-year, $275 million contract that has become a burden for the Yankees. Major League Baseball officials remain uneasy about his past links, however indirect, to performance-enhancing drugs, unsure if they know all the facts. And the Yankees appear to share some of those sentiments.

That uneasiness came into play in December, when the Yankees learned that Rodriguez needed to have surgery on his left hip less than four years after he had a similar procedure on the right one. At that point, the Yankees made clear to Rodriguez that they did not want him to go back to the doctor he used the first time, according to several people in baseball briefed on the matter.

That doctor, Marc Philippon, has had links to Anthony Galea, the Canada-based physician who secretly treated Rodriguez as he recovered from his 2009 operation. Galea pleaded guilty two years later to federal charges stemming from his distribution of human growth hormone to professional athletes.

Although Rodriguez insisted in an interview with investigators for Major League Baseball that he never received banned substances from Galea, those investigators have not been satisfied that they know the whole story. On more than one occasion, they have been rebuffed when they asked federal authorities to share evidence related to Galea’s treatment of Rodriguez.

But while that issue appears to be at a standstill, baseball’s investigators are now examining what links Rodriguez and several other players may have with a Miami-area man named Anthony Bosch, who has previously come to the attention of baseball and federal authorities.

According to several people in baseball, the investigators are again looking at Bosch and his father, Pedro, a physician, who came under the scrutiny of baseball and the Drug Enforcement Administration in 2009. At the time, baseball’s investigators suspected that Manny Ramirez, who had been suspended for 50 games for violating baseball’s drug-testing program, had received a banned substance from Pedro Bosch.

The new interest in the Bosches was first reported Saturday by The Daily News, which said that Anthony Bosch had advised Rodriguez on his nutrition and physical training and had helped him get input on blood test results.

It is unclear just how seriously federal authorities are scrutinizing either of the Bosches or whether any action can eventually be taken by Major League Baseball, which has struggled for several years to punish players who have been linked to performance-enhancing drugs but have not tested positive for a banned substance.

The latest revelations come as Rodriguez continues to recuperate from the operation he had in New York on Jan. 16 to repair a torn labrum and impingement in his left hip. Dr. Bryan Kelly, an orthopedist at the Hospital for Special Surgery,