They aren't required by law to store the information for any specific amount of time, so they set their own policies governing the deletion of files.
Roberts said that from his experience, ISPs tend to delete IP information after about six months; Web companies, such as Google, tend to hold on to information for longer.
The information held by the ISP, however, is actually more critical, he said. While an IP address stored by a website might be able to lead a forensic expert to the ISP that enabled the Internet connection, the IP address held by an ISP can actually lead the expert to an Internet troll's front door.
But Internet companies don't just hand over these keys. A subpoena and, in some cases, a court order, are necessary to force the companies to release the IP addresses.
Even after Roberts obtains the requested IP addresses, his job is not always finished.
"So many cases fail, whether it be harassment, wire fraud, whatever it might be, because it's like a doorway we know the person walked through, but so did 100 other people," he said.
An IP address can tell him where the poster's computer may be located, but it doesn't absolutely link a specific person to that computer.
To identify the individuals behind online attacks, Roberts said he uses a combination of high-tech expertise and old-fashioned instinct.
"We have to triangulate the individual with digital evidence that can't be obtained electronically," he said.
Assuming a victim has a sense of who might be behind the attacks (which they often do, he said), he and his specialized team reach into their black box of tricks to map out the individual's social network.
Using what he called "social forensics," they figure out the person's friends, most frequented sites and habits online.
"They leave lots of electronic footprints," he said.
In 75 to 85 percent of the cases, those tactics are enough to positively identify the perpetrators of online attacks, Roberts said. But in some cases, he goes a step further, enlisting a Ph.D.-trained forensic linguist.
"[She] can examine known samples of writing with the anonymous writings and link them together with the little idiosyncrasies in the person's prose," he said.
While Roberts does take on several cases on a pro bono basis, he said, most cases cost about $5,000 to $10,000 (not including legal fees which, depending on the circumstances, can significantly inflate the price).
But his clients say that the expense is worth it.
Patty McPeak, founder of natural products company Nanacea, said that after she left her previous company, where she had been CEO, online commenters wrote vicious posts about her and her family.
The ordeal continued for about two years, but Roberts' company helped expose the commenters. And when McPeak let them know that she had learned their identities, the online onslaught stopped.
At 69-years-old, she said, she had no desire to spend her golden years suing them, but just knowing who was behind the attacks was enough.
"I would have gone to my grave wondering who would have said such awful things about me," said McPeak, who lives in El Dorado Hills, Calif. "I have such peace of mind knowing who these people are. It would never have been possible without this."