There is convincing evidence that the formation of complex organic molecules occurred in a variety of environments. One possible scenario highlights the universe as a giant reactor for the synthesis of organic complex molecules, which is confirmed by numerous identifications of interstellar molecules. Among them, precursors of biomolecules are of particular significance due to their exobiological implications, and some current targets concern their search in the interstellar medium as well as understanding the mechanisms of their formation. Hexamethylenetetramine (HMT, C₆H₁₂N₄) is one of these complex organic molecules and is of prime interest since its acid hydrolysis seems to form amino acids. In the present work, the mechanism for HMT formation at low temperature and pressure (i.e. resembling interstellar conditions) has been determined by combining experimental techniques and DFT calculations. Fourier transform infra-red spectroscopy and mass spectrometry techniques have been used to follow experimentally the formation of HMT as well as its precursors from thermal reaction of NH₃:H₂CO:HCOOH and CH₂NH:HCOOH ice mixtures, from 20 K to 330 K. DFT calculations have been used to compute the mechanistic steps through which HMT can be formed starting from the experimental reactants observed in solid phase. The fruitful interplay between theory and experiment has allowed establishing that the mechanism in the solid state at low temperature is different from the one proposed in liquid phase, in which a new intermediate (1,3,5-triazinane, C₃H₉N₃) has been identified. In the meantime, aminomethanol has been unambiguously confirmed as the first intermediate whereas the hypothesis of methylenimine as the second is further strengthened.