MINNEAPOLIS -- As the Minnesota Vikings approach the close of a busy first week of free agency, they're not done looking at possible additions to the team, general manager Rick Spielman said on Friday.

Where the Vikings go from here could take a more measured approach, given the fact the team has about $16 million in cap space left and needs to reserve some money to sign draft picks, as well as for other contingencies that could come up. But the Vikings have several players coming in for visits next week, Spielman said, and could still add to their roster from the open market.

"We'll continue to monitor the market. We'd like to still fill some holes," Spielman said. "We're not, by any means, saying we're completely finished. You don't know what's going to happen. There are some guys looking for a lot of money at this point, and a week from now, that could potentially change."

As the Vikings pick through the market, they've still got several ways they could go on defense. Former Chicago Bears defensive tackle Henry Melton visited the Vikings on Thursday, and was scheduled to head to Seattle and Dallas next. "You've got to go with the pace of the player, too," Spielman said.

One player who would likely be affected by Melton is defensive tackle Kevin Williams, who has talked with the Vikings about returning for a 12th season but appears to be in a bit of a holding pattern at the moment. If the Vikings signed Melton, they might not have room for another three-technique tackle like Williams.

"We talked with his agent a few days ago," Spielman said. "I told him we'd leave it open, but we were trying to address some other needs that we definitely wanted to get done first. There's no decision that's been made on Kevin Williams at this point."

Spielman said the Vikings are working to bring back left guard Charlie Johnson, who has received interest from several teams, and said he's fine at the moment with the Vikings' linebacker depth, adding the Vikings could use linebacker Chad Greenway in some different ways than he's been used in the past.

"I know he's played on the weak side in our nickel package and stuff," Spielman said. "They do a lot of things from a defensive side that are a little different than we've done in the past."