Story highlights Suspects were making the explosive used in Paris and Brussels, a source tells CNN

The arrested teenage girl pledged ISIS allegiance in video, a source says

(CNN) Suspects arrested Friday in a foiled terrorist plot in France had just started making the same powerful explosive used in the ISIS-directed Paris and Brussels attacks, and they appear to have been inspired by the terrorist group, a source close to the investigation told CNN.

French police "thwarted an imminent attack on French soil" when they arrested four people, including a 16-year-old girl and three men, in cities across France, Interior Minister Bruno Le Roux said in a statement. The girl had pledged allegiance to ISIS in a cellphone video, the source said.

All four suspects are French nationals, a source familiar with the investigation said.

A partially assembled improvised explosive device was also found as part of the investigation, according to Le Roux's statement.

When they were arrested, the plotters had just begun making TATP, the same explosive used in Paris and Brussels, the source tells CNN. TATP, or triacetone triperoxide, is a chemical powder made from common household objects that can explode when subjected to heat or friction.

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