LIVERMORE — Ross Dmochowski lives online, where he engages in sometimes serious, sometimes juvenile, sometimes offensive political discussions about President Barack Obama.

But Dmochowski, a 42-year-old tech professional, was unprepared when his online chats resulted in a bomb threat under his name that brought FBI agents to his Livermore home — and contributed to his getting fired in September from his $120,000-a-year job at Yahoo.

While online harassment is hardly rare, what’s surprising about Dmochowski’s experience is that even a savvy denizen of the Web world can underestimate how Internet conversations can quickly upend life both at home and at work.

“Online interactions that turn sour can be life-changing,” said Eric Goldman, a Santa Clara University law professor and director of the university’s High Tech Law Institute. “Negative online encounters can happen to anyone, just like the most skilled professional driver can have a car accident on the roads due to other people’s driving mistakes.”

In fact, despite his tech background, Dmochowski did little to protect himself, other than blocking some of the more offensive posters.

He uses his real name on his Google+ account, which has nearly 10,000 followers. And, despite pleas from his fiancee and his recent experience, Dmochowski still refuses to temper his opinions or step away from nasty exchanges.

“The Internet is my home,” he defiantly told this newspaper. “This is me standing my ground.”

But several of Dmochowski’s online friends have limited their Google+ interactions because of harassment from the same people who apparently linked Dmochowski to the bomb threat.

The aggressors, Dmochowski and his online friends said, are all white men from various states. Among other things, the men turned on a couple of black women, repeatedly calling them the “N-word” online and using harsh sexual terms. They also attacked a gay African-American man with both racist and homophobic comments.

Dmochowski and others said he became the primary target in early September when he objected to attacks against one friend that included a threat to use Photoshop to put porn on images of the man’s wife and children and a post that said, “I know your kids’ bus route and I know where you live.”

The attacks on Dmochowski included a photo of his face behind cross-hairs and a picture of him holding hands with his then-2-year-old daughter, whose face was doctored with a Ku Klux Klan hood and the letters “KKK” on her dress.

Dmochowski, an unapologetic liberal Democrat and Obama supporter, insists that he never threatened or harassed anyone. But he does acknowledge name-calling, such as “your mother’s a hillbilly whore.” A review by this newspaper of his Google+ posts also showed Dmochowski sometimes uses a homophobic slur, which the people he engages with also use regularly.

Dmochowski said he only uses the slur to highlight the “hypocrisy” of his attackers.

Less than three weeks after objecting to the threats to his friend, on Sept. 23, Dmochowski was fired from his job as a senior engineer revamping Yahoo’s home page. He said one of his online antagonists had doctored a Google+ post to create an online bomb threat in Dmochowski’s name that was directed at Fox News.

The threat caused the FBI to contact Yahoo, then send a pair of agents to Dmochowski’s home. The FBI declined to comment.

While he has not been arrested or charged, Dmochowski said that Yahoo told him he was fired for using social media on his Yahoo laptop.

Yahoo spokeswoman Sarah Meron confirmed that Dmochowski no longer works for the company.

Yahoo does not prohibit employees from using social media, Meron said, declining to specifically discuss his firing. She instead referred a reporter to Yahoo policy language that states: “If you decide to post complaints or criticism, avoid using statements, photographs, video or audio that reasonably could be viewed as malicious, obscene, threatening or intimidating, that might constitute harassment or bullying, or are slanderous or detrimental to Yahoo.”

In February, Dmochowski filed a Livermore police report asserting that a man named W. James Hamel claimed online that he had photos of Dmochowski sexually abusing his daughter and posted that Dmochowski “seriously needs a lead pipe buried in his skull.”

Hamel, a database administrator from Navarre, Fla., said in a brief interview that Dmochowski “was stalking me and a number of other people for about a year.”

Hamel then hung up and did not respond to messages seeking further comment. Hamel did write a profanity-filled post about the interview on his Google+ account, which is dominated by a photo of Nazi SS chief Heinrich Himmler and other German officers walking past an emaciated soldier inside a World War II prison camp.

“These guys use all kinds of tactics on social media to silence people,” Dmochowski said. “But I won’t back down. That’s what really bugs them.”

Google declined to discuss Dmochowski’s claims but said it takes reports of impersonation and harassment “very seriously. We have strict policies in place to remove this type of content and ban users we find in violation of our policies. We encourage the Google+ community to help identify and flag inappropriate content.”

While Livermore police detectives regularly get reports of social media bullying among teenagers who know one another, Sgt. Paul Mayer called Dmochowski’s police report, involving accusations from an adult victim with an adult suspect across state lines, “a first for us.”

Mayer, who is in charge of the Livermore Police Department’s detectives, said his department does not have the staff to investigate Dmochowski’s accusations out of state. But Mayer forwarded the case to local sheriff’s deputies in Florida and to the FBI.

Law enforcement agencies have become much more aware of online harassment, but victims’ complaints rarely lead to an arrest, Goldman said.

To make an arrest, police and prosecutors have to distinguish between a credible, “actionable threat” and “innocuous comments” that are protected by the First Amendment, Goldman said.

“Just because it sounds threatening doesn’t mean it’s actually criminally threatening,” he said.

Dmochowski has since gotten a new tech job in Silicon Valley — and continues to post political comments on Google+.

“My fiancee wants me to just walk away,” he said. “If I did that, I would have problems looking at myself in the mirror. Free speech is not absolute. And eventually they’re going to pick a fight with the wrong guy.”

Contact Dan Nakaso at 408-271-3648. Follow him at Twitter.com/dannakaso.