Danish authorities say a deadly accident which saw a high-speed passenger train strike a semi-trailer that fell off a freight train coming from the opposite direction, "very likely" happened because the unit "wasn't properly secured."

The Accident Investigation Board said Thursday in a preliminary report that "local wind conditions could have had enough strength" to knock semi-trailer off the freight train's flatcar as it crossed a bridge and tunnel link between central Denmark islands.

Eight people were killed and 16 injured in the Jan. 2 accident on a bridge during strong winds. Victims were all on the passenger train.

The freight train involved in Denmark's deadliest train accident in 30 years was transporting semi-trailers filled with beer crates.