In the weeks before they carried out an attack in the Spanish city of Barcelona last summer, a cell of jihadists inspired by the Islamic State traveled repeatedly to France, where they bought a camera and recorded footage of the Eiffel Tower.

That video, as well as the stunning quantity of precursor chemicals used to make explosives which were recovered from the safe house they used, has led experts to conclude that the terrorists were plotting something much larger and more catastrophic. The materials they left behind suggest that their plan involved packing vans with explosives and attacking targets not just in Spain, where the group killed 16 people and injured 140 others in August, but also possibly in France.

“The existing evidence indicates the terrorist cell had planned a far more ambitious and potentially more deadly operation,” Fernando Reinares and Carola García-Calvo of the Program on Global Terrorism at the Elcano Royal Institute in Madrid wrote in a study published on Wednesday.

“Considering the lethal resources assembled by the terrorists and their lethal intent, the death toll could have reached hundreds had they not accidentally blown up their bomb factory,” said the report, which appeared in the CTC Sentinel, the publication of the Combating Terrorism Center at West Point. It is based on interviews with investigators, a review of court proceedings and an analysis of videos, images and other materials recovered at investigation scenes.