Washington Wizards’ Jason Collins, left, goes to the basket against Chicago Bulls’ Taj Gibson during a NBA basketball game in Chicago on April 17, 2013. (Jim Young/Reuters)

As Jason Collins, a 7-foot journeyman center for the Washington Wizards, played out the final few weeks of his 12th National Basketball Association season this spring, he was also finalizing plans for an announcement that would send shock waves across the world of sports. That announcement came late Monday morning, with the Internet publication of a first-person story in Sports Illustrated:

“I’m a 34-year-old NBA center,” the story began. “I’m black. And I’m gay.”

With that, Collins became the first active male athlete in a major U.S. professional sports league to come out of the closet — a designation that is certain to elevate this relatively anonymous player, known primarily for his ability to commit fouls and set picks, into a historic figure in both the sports and gay-rights realms.

“I didn’t set out to be the first openly gay athlete playing in a major American team sport,” Collins wrote. “But since I am, I’m happy to start the conversation. I wish I wasn’t the kid in the classroom raising his hand and saying, ‘I’m different.’ If I had my way, someone else would have already done this. Nobody has, which is why I’m raising my hand.”

The first wave of public reaction to Collins’s announcement was overwhelmingly positive. President Obama called Collins “to express his support and said he was impressed by his courage,” according to a White House official. Los Angeles Lakers star Kobe Bryant, former NFL star and current daytime TV talk host Michael Strahan and former president Bill Clinton were among others who issed public salutes to Collins.

Even amid widespread speculation that one or more male athletes would soon break the sexuality barrier — former Ravens safety Brendon Ayanbadejo suggested in recent interviews that one or more NFL players were preparing to come out — Collins’s announcement stunned people around the NBA, including his Wizards teammates. Several of them said they had no inclination Collins was gay.

“My first reaction was I felt for him,” said Wizards center Emeka Okafor, who said Collins, his backup, called to give him the news shortly before the article went live. “I was like, ‘Wow, you’ve had to carry this around for so long.’ I can only imagine the emotional toll that it must take, and also the strength it must take to come out. . . . Maybe more people will [come out] now. Maybe this will be the spark where other people feel comfortable.”

Until recent years, the notion of an openly gay male athlete was thought to be a near-impossibility in major U.S. professional sports, where the testosterone-fueled culture of the locker room often bordered on homophobia. While celebrated female athletes such as tennis player Martina Navratilova and basketball player Sheryl Swoopes were comfortably out of the closet during their careers — and several male athletes revealed their homosexuality after their playing careers — no active male athlete could afford to take a similar step.

As recently as 2002, baseball star Mike Piazza felt compelled to call a news conference to refute rumors he was gay. In 2007, former NBA all-star guard Tim Hardaway said in a radio interview: “I don’t like gay people and I don’t like to be around gay people. I am homophobic.” In 2011, the Lakers’ Bryant was fined $100,000 for using a homophobic slur in a rant against a referee.

But in recent months, the widespread gains in acceptance toward gays in American society as a whole began to trickle down to one of the last frontiers of intolerance — the major U.S. sports leagues. Amid growing speculation that an active athlete would soon come out of the closet, the NBA, National Football League, National Hockey League and Major League Baseball began making preparations for it. The NHL, for example, partnered with the You Can Play Project — an advocacy group for gay athletes — to begin counseling and training personnel for the eventuality.

In some regards, Collins may have been the perfect athlete to break this barrier. As a fringe player in his sport who is nearing the end of his career, he has little social capital — in the way of commercial endorsements and job security — to risk, and his reputation for hard-nosed toughness on the court could help deflect ridicule. His eloquent, first-person account of his decision could sway public opinion in his favor; he was scheduled to appear on ABC’s “Good Morning America” on Tuesday.

The D.C.-based advocacy group Human Rights Campaign went so far as to equate Collins with Jackie Robinson, who famously broke baseball’s color barrier in 1947 and is the subject of a current biopic.

“It’s a tremendous breakthrough,” said Michael Cole-Schwartz, a spokesman for the group. “If you’re in professional sports and in the public eye, it’s understandable that there will be some fear of what happens next. By Jason taking this step, he has proven to others that the reality is far better than those fears.”

Collins said he was inspired to come out of the closet, in part, by Rep. Joe Kennedy (D-Mass.), who was Collins’s roommate at Stanford. After Kennedy informed Collins he had marched in the 2012 Gay Pride Parade in Boston, Collins was envious. “I was proud of him for participating,” he wrote, “but angry that as a closeted gay man I couldn’t even cheer my straight friend on as a spectator. . . . I want to march for tolerance, acceptance and understanding. I want to take a stand and say, ‘Me, too.’ ”

Collins came to the Wizards, his sixth NBA team, on Feb. 21 via a trade with the Boston Celtics, and played in only six games for Washington in March and April, scoring a total of four points. But during that time, he began preparing to come out. According to a note from a Sports Illustrated editor about how Collins’s story came together, the player’s agent contacted the magazine for a possible collaboration in late March. The first-person story appeared 12 days after the conclusion of the Wizards’ season.

During his time with the Wizards, Collins explained his unusual uniform number — 98 — as being designed to make life difficult for the referees, who signal uniform numbers to the scorer’s table when calling fouls. But in his article, Collins explained the real reason for his choice of 98: as a tribute to Matthew Shepard, the University of Wyoming student who was tortured and murdered in 1998 in one of the most notorious anti-gay hate crimes in recent history.

“When I put on my jersey,” he wrote, “I was making a statement to myself, my family and my friends.”

Collins became a free agent at the end of the Wizards’ season, and the NBA talent marketplace doesn’t usually look kindly upon 34-year-old, end-of-the-bench players. Collins says he wants to continue playing, but it remains to be seen whether Collins can find work for the 2013-14 season — and whether Monday’s announcement helps or hurts that effort.

“How I think of him doesn’t change. I still would love to have Jason part of our team,” Wizards Coach Randy Wittman said. “Black, white, Jewish or Christian. Religion, sex. It’s all the same. Who gives anybody the right to judge anybody?”

Carol Morello and Zachary A. Goldfarb contributed to this report.