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CNN) -- Police are asking the public for help in finding two Milwaukee, Wisconsin, boys who disappeared last Sunday.

"We know there's additional information out there," Milwaukee police spokesman Anne E. Schwartz said Saturday. "If someone has provided you with that information, we need you to give us a call."

Quadrevion Henning, 12, and his friend Purvis Parker, 11, were last seen Sunday afternoon as they headed off to play ball at a park near their homes. Extensive searches and numerous tips have yielded no clues to what might have happened to them.

Police are confident, however, that there are people with information who have not come forward.

"If you've been considering calling us with this information, if you're afraid, now is the time. We need you to give us a call," Schwartz said, asking anyone who may have been near the park between 3 p.m. and 5 p.m. Sunday, "Where were you? What were you doing? What did you see?"

"This is six days, six days that these boys have been missing," she said. "If you think somebody else called, if you think your information isn't important, you're wrong,"

Calls to the police's nationwide tip line can be made anonymously, she said.

"We're putting this out as a general plea," Schwartz said. "Every little tip is a big tip."

Hundreds of tips have poured in, and local businesses have joined the effort, putting the boys' pictures on everything from pizza boxes to billboards, she said. Reward money had reached $35,000 in public and private donations by Saturday afternoon.

Henning is 5-foot-4 and weighs about 110 pounds. He was last seen wearing a black hooded pullover and black jeans.

Parker is 5-foot-1, 120 pounds and was last seen wearing a gray, hooded jacket, gray jogging pants and black tennis shoes.

Police are operating on the belief that Henning and Parker are still in Milwaukee, she said.

Asked if authorities believe they are safe, she said: "What we're hoping to do is get enough information so we can find the boys right now. We don't know where they are. Until we know where they are, we can't say that they're safe."

Both Henning and Parker are reported to be good students and it is not likely that they ran away, according to authorities and family members.