The Bills announced Wednesday evening that Williams had left their facility and would return Thursday morning.
Williams' agent, Ben Dogra, wrote in a text to The Associated Press: "We are still continuing to talk."
That means the former Houston Texans star pass-rusher will sleep on deciding if he wants to be a Buffalo Bill. The Bills flew him into town by private jet from his home in North Carolina shortly after the NFL's free-agency period opened on Tuesday.
Williams had informed the Bills on Tuesday night that he planned to visit other teams, two sources told ESPN.com's John Clayton.
Despite failing to make a decision, it was a busy day for Williams a day after he had dinner with Bills management and coaches at a downtown Buffalo restaurant. After taking a physical and touring the Bills' facility on Wednesday morning, Williams traveled to the Buffalo International Airport to pick up his fiance.
Williams told Buffalo's WIVB-TV, "We'll see," when asked if his fiance's arrival was considered a good sign regarding his negotiations with the Bills.
The Bills said Williams and his fiance, who was not identified, were given a tour of the region.
Defensive tackle Kyle Williams, who was at Tuesday night's dinner, told a radio station he's optimistic the Bills will sign the former North Carolina State product.
"I have a good feeling about it," Kyle Williams told WGR 550 radio, according to the team's website. "We were together 'til 11 at night at the facility hanging out having a good time. The powers that be were working late last night, so I feel good about it. I'll see him again (Wednesday) morning at the facility again and the fact that he's still here is good. Now we just need a couple hundred people to lay down on the tarmac at the airport."
Behind quarterback Peyton Manning, Mario Williams is regarded as one of the top free agents available. And that means the Bills would have to pay a hefty price if they intend to land the player selected first overall in the 2006 draft by Houston.
The Bills have identified improving their pass rush as a top offseason priority after managing just 29 sacks -- including 10 in one game -- last season.
Listed at 6 feet 6, 285 pounds, Mario Williams is a two-time Pro Bowl starter, who had a Texans' franchise-best 53 career sacks, including a team single-season best 14 in 2007. Williams was limited to playing just five games last season as a result of a torn chest muscle. He also missed three games in 2010 after having surgery to repair a sports hernia.
Kyle Williams said he told the former No. 1 overall pick that if he signs with Buffalo, he could make the Bills' defensive line, which also boasts 2011 first-round draft pick Marcell Dareus, the best in the league.
"I think he wants to go somewhere where he can get somebody over the hump and put a team on the next level," said Kyle Williams, according to the team's website. "And I told him he could be that guy. I told him point blank, 'If you're here and everything goes right, we'll have the best D-line in the league.' I told him that because I honestly believe that."
Aside from courting Williams, the Bills also brought in free agent receiver Robert Meachem on Tuesday. Meachem, who spent his first six seasons in New Orleans, turned down the Bills, signing with the San Diego Chargers.
With speculation that Williams was attending the Buffalo Sabres' NHL game against Colorado on Wednesday, several "We want Mario!" chants went up in the arena.
Sabres spokesman Michael Gilbert said he did not know if Williams was in attendance.
A business not far from Ralph Wilson Stadium had "Welcome Mario Williams" flashing on its electronic sign. A couple of fans dressed in Bills jerseys waited to greet Williams after his dinner on Tuesday. And several fans have showed up in the parking lot of the Bills facility hoping to get a glimpse of Williams the past two days.
Even Hall of Fame quarterback Jim Kelly's wife, Jill, got into recruiting Williams via her Twitter account on Wednesday. Noting Williams' passion for hunting, Jill Kelly tweeted that there were currently nine deer in her backyard in suburban Buffalo.
Information from ESPN.com senior NFL writer John Clayton and The Associated Press was used in this report.