Schools in this town, which last week saw one of the worst massacres in United States history, opened their doors again Tuesday, except for Sandy Hook Elementary, scene of the tragedy.

All Newtown schools were closed as a safety precaution while last Friday's shooting was being investigated.

Four days after 20-year-old Adam Lanza broke into Sandy Hook and killed 20 children and six adults before committing suicide, the building remained closed.

Lanza had previously gunned down his own mother.

State police Lt. Paul Vance said the crime scene investigation could take months. Nor would he rule out the possibility that the school might never reopen.

The great enigma that investigators have not been able to clear up is why Lanza did it. He left no previous notes or letters indicating why he would kill his mother and then shoot up a class full of children at a school with which he apparently had no connection.

Meanwhile, students from Sandy Hook are using the facilities of Chalk Hill, a vacant middle school in the nearby town of Monroe.

Newtown police plan to station cops in the six schools that reopened Tuesday morning in order to provide security for the more than 4,700 students who have returned to class. EFE