Safety board spokesman John Cottreau said the evidence gathered in community of Wapske would be assessed and officials would then decide the extent of their investigation.
"Depending on what that decision is, that will determine what our next steps will be," said Cottreau on Sunday.
CN (TSX:CNR) spokesman Jim Feeny has said investigators were looking at the possibility that a wheel or axle problem on a freight car was to blame, but no conclusions have been reached.
About 150 people who had to leave their homes last Tuesday night when the 122-car train derailed were allowed back Saturday, with the exception of one couple whose house is within 100 metres of the derailment.
A CN crew was finally able to extinguish flames overnight Friday that ignited when 19 cars of the freight train jumped the tracks.
The first freight train passed through the derailment site two hours after residents returned to their homes.
Feeny said crews made progress in cleaning up the site on Sunday.
"Cleanup is progressing as planned, including the beginning of the transfer of the four remaining tankers that are on the site," said Feeny.
It wasn't known when the last couple would be able to return to their home, he said.
The rail company is promising to compensate those whose properties were damaged and had expenses due to the evacuation.
The province's Health Department was asking people in the area with private wells not drink their water until after they have been tested. Feeny said water was being tested on Sunday, although he couldn't say when the results would be known.
The Canadian Red Cross said it was winding down its operations in Wapske on Sunday.
— By Aly Thomson in Halifax