CONCORD, N.H. -- Investigators are continuing to search for the bodies of two missing Concord children, as their family prepares for a memorial service Thursday.

Philip and Sarah Gehring are still missing, and authorities are searching the Midwest for the site where they were allegedly buried by their father, Manuel Gehring.

Investigators in New Hampshire continue to analyze the evidence they've collected, but organized searches for the bodies ended weeks ago. Authorities said the number of tips has also dwindled.

"We're trying to look at different strategies to continue with the search," Assistant Attorney General Jeff Strelzin said. "There's been some consideration about having teams out in connection with our agencies and other groups. There have been volunteers, and we're looking at whether or not that would happen in the future."

Ohio police said they are examining every exit along Interstate 80 in that state. Authorities believe the children's father shot them in New Hampshire and buried them somewhere in the Midwest.

The children's mother, Teresa Knight, has been in seclusion since the children disappeared. Wednesday, the attorney who handled the Gehrings' divorce spoke on camera for the first time since the story appeared.

"I know it's been a difficult time for Teri," attorney David Phillips said. "Hopefully the memorial service will be a way to address the memory of their children."

The memorial service for the Gehring children will be held Thursday morning at the South Congregational Church in Concord at 11 a.m.