NEW YORK—Prosecutors in the Silk Road drug-trafficking trial have shown heaps of evidence from the laptop they seized from Ross Ulbricht, the man they say was the kingpin behind the world's biggest drug-trafficking website.

What hasn't been known, until this morning, is what exactly led them to Ulbricht. A Homeland Security agent who testified last week had been investigating the site since it became famous in mid-2011, but it wasn't until September 2013 that he heard the name "Ross Ulbricht." Days later, Ulbricht had been arrested.

Today, IRS Special Agent Gary Alford took the stand and explained how he got onto Ulbricht's trail using one of the most basic tools on the Internet: a simple Google search.

Even though the Silk Road operated only on Tor, the part of the Internet sometimes referred to as the "darknet," Alford knew it had to be marketed somewhere.

"I figured it had to be on the regular Internet so someone could tell you where to go," Alford said on the stand.

In June 2013, he searched for "Silk Road" and ".onion," looking only at Web references that pre-dated January 31, 2011—the very beginning of the market. Tor-only websites have an extension of ".onion" rather than the usual ".com" or ".org" extensions. The search led him to a post on bitcointalk.org called "A Heroin Store," which is still up.

The post quoted a January 29 post—since deleted—from a user named "altoid."

"What an awesome thread!" wrote altoid. "You guys have a ton of great ideas. Has anyone seen Silk Road yet? It's kind of like an anonymous amazon.com. I don't think they have heroin on there, but they are selling other stuff. They basically use bitcoin and tor to broker anonymous transactions."

Those interested, altoid continued, could visit silkroad420.wordpress.com, where they'd find instructions on how to get to Silk Road.

Alford searched all of "altoid's" posts. The same user had written in an October 2011 post that he was seeking "the best and brightest IT pro in the bitcoin community to be the lead developer in a venture backed bitcoin startup company." The recruitment post suggested sending an e-mail to "rossulbricht at gmail dot com."

Altoid had also made one post on shroomery.org, a forum about mushrooms. His post on drugs-forum.com was rejected as spam. Like the others, it was written from the perspective of a user simply "checking out" the Silk Road.

Soon enough, Alford had a search warrant to go through the rossulbricht@gmail.com e-mail account. In it, he found all kinds of data—which the government has just started to show the jury—that line up with the data found on Ulbricht's computer.

In the Gmail account, Alford found e-mails to a man in Bastrop, Texas, seeking to rent out his house. On Ulbricht's computer, there's a journal entry describing how he went to Bastrop to grow psychedelic mushrooms. Items found on the Silk Road expense spreadsheet, include a "clean room HEPA filter" for $89.54 and Ulbricht's $1,150 Samsung 700z laptop. Alford found corresponding receipts for those items, from Amazon, in Ulbricht's Gmail.

The Gmail account also reveals more about Ulbricht's travels. He bought an itinerary through CheapAir.com, which took him from Austin, Texas, to Sydney, Australia, in October 2010. The return ticket from Sydney to Austin, which had a stop in San Francisco, was for April 2012.

During that time, he also traveled in Asia. In late January 2012, Ulbricht wrote a Facebook post that said, "Surprise, I'm in Thailand now :)... Hanoi was way too cold and the allure of a warm beach was way too much."

Alford was only on the stand for about 90 minutes today, as the jurors were released at noon due to the impending blizzard in New York. The trial is set to continue on Thursday. Alford's testimony seems to give the defense even more of a hill to climb.

Today was the seventh day of the Silk Road trial. For other trial coverage, see our series page.