In the wake of an accident in which a 6-year-old boy was swallowed by a sink hole in the Mount Baldy sand dune at Indiana Dunes National Lakeshore, officials have closed access to the massive dune indefinitely.

The youth, who was rescued after being trapped for three-and-a-half hours, was reported to be in critical condition Monday at a Chicago hospital.

Lakeshore officials say the continued closure of Mount Baldy is needed to bring in additional resources for the investigation of the hole that opened up last Friday.

"Based on evidence gathered at the scene, park officials are looking into the possibility that the hole was created by a long-buried tree that had decomposed, leaving a void in the sand," said Supervisory Park Ranger Bruce Rowe in a release. "While no such occurrence has ever been observed in the park, officials plan to bring in ground-sensing equipment to look under the surface of the dune to see if voids or other potential hazards can be identified."

Additional expertise was being sought from the Park Service's Geologic Resource Division. A safety protocol was also being developed before park staff or researchers are allowed onto Mount Baldy.

The closure includes Mount Baldy, its parking lot, trails and beach area. Signs are posted warning of the closure. Anyone entering the closed area is subject to law enforcement action.