MARQUETTE, Mich. -- Actor John Stamos testified Tuesday he did not use cocaine or behave inappropriately six years ago with an underage girl now accused of trying to squeeze $680,000 from him by threatening to sell compromising photos to celebrity tabloids.

Testifying in U.S. District Court, the "Full House" and "ER" star acknowledged having a "flirty" e-mail relationship with defendant Allison Coss after socializing with her in 2004, but denied knowing she was 17 at the time.

Stamos, 46, also denied defense lawyer Sarah Henderson's accusation that he stripped and sat in a hot tub in his hotel room with Coss, who was wearing only underclothes. And he insisted knowing all along that no photos of him using illegal drugs existed, despite corresponding online with someone claiming to have such pictures.

Coss, now 24, and 31-year-old Scott Sippola are charged with conspiracy and two counts of extortion. If convicted, the Marquette couple could get up to five years in prison on the conspiracy charge and two years on each extortion count.

Questioned by prosecutors, Stamos said he met Coss in Orlando, Fla., in 2004 during a visit with three male friends trying to cheer him up following his separation from his wife, actress and supermodel Rebecca Romijn.

"I was very heartbroken at the time," he said.

Stamos said he met Coss at a dance club where patrons must be at least 18 and she told him she was a college student on spring break. She actually was a high school student who had used a borrowed ID to get into the club, Henderson told jurors during opening arguments Monday.

They didn't spend much time together that night, Stamos said, but he got her phone number. The next day, they went to Disney World and later to his hotel room, where up to 10 people had gathered.

"We were just hanging out, socializing," he said.

Henderson said during her opening remarks that the night Stamos and Coss met, they wound up in his hotel suite with other people, including two strippers with cocaine. Henderson said Stamos was photographed bending over a table where the drugs had been laid out and that he offered to perform oral sex on Coss when they were on his bed kissing, but she declined.

Stamos was not asked about the alleged sex offer during his testimony, and he did not address it. The presiding judge, R. Allan Edgar, last week ruled that questions about a sexual relationship between Coss and Stamos would not be allowed.

But under questioning by Henderson, Stamos said the hot tub allegation was untrue and insisted he did not use cocaine.

He said after the Florida trip, he began communicating with Coss via e-mail and described their conversations as "flirty" but "friendly."

"It was all very sweet. I considered her a friend," Stamos said.

He acknowledged asking for "hot" photos of Coss by e-mail in 2007. "Send me some wild pics if you're so wild, wild child," one of his messages read.

Prosecutors say the extortion attempt occurred last fall. They had Stamos read from e-mail exchanges, beginning with two messages from "J Taylor" who claimed he had gotten her pregnant.

"If this gets out, I am screwed. But most of all you are screwed 10 times more than me," it said. Henderson acknowledged Monday that Coss had sent the message.

Afterward, Stamos testified, Coss began sending him e-mails saying a man was harassing her about having incriminating photos of her and Stamos.

For the most part, Coss appears more fearful than Stamos in the e-mails. "I'm not worried about pics," he says. "The worst we were doing was drinking and goofing off." In another message, he says of the man threatening Coss: "I'll bust him up. ... We didn't take any bad pics. I'm too smart."

In November, Stamos begins communicating with "Brian L," who prosecutors say was Sippola. The man says the photos contain images of drug-taking, alcohol use and strippers. Brian L also says he's been offered $780,000 in a tabloid bidding war and offers the pictures to Stamos for $680,000, according to the e-mail read by Stamos in court.

Stamos said around this time he contacted the FBI: "I felt threatened, violated. I felt this was illegal."

He said FBI agents began advising him what to tell Brian L and eventually handled the correspondence themselves while developing a sting operation. It ended Dec. 2 with Coss and Sippola being arrested during a phony money drop at K.I. Sawyer International Airport near Marquette, in Michigan's Upper Peninsula.

Defense attorney Frank Stupak grilled Stamos about why he repeatedly asked Coss and Brian L what the alleged photos showed if he knew there was nothing to fear. Stamos said someone might have doctored photos with a computer.

Stupak also asked whether photos of Stamos with alcohol and women under 21 would be bad for his image.

"By the way, are you a friend of Charlie Sheen?" Stupak asked.

Stamos looked amazed, but quickly replied, "No, but I know Lindsay Lohan." The courtroom erupted in laughter.