Short-range spin freezing in the double trillium lattice spin-liquid candidate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>KSrFe</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mrow> <mml:mo>(</mml:

The authors investigate here the double-trillium lattice antiferromagnet KSrFe${}_{2}$(PO${}_{4}$)${}_{3}$ using ${}^{31}$P NMR, thermodynamic measurements, and first-principles calculations to elucidate its unconventional ground state. Below the characteristic temperature $T$* ~ 3.5 K, the results reveal a short-range spin freezing state with persistent antiferromagnetic fluctuations, contradicting earlier proposals of a gapless spin-liquid state. A nearly field-independent NMR linewidth, strong enhancement of 1/${T}_{2}$ below $T$*, and sizeable antiferromagnetic exchange interactions collectively position KSrFe${}_{2}$(PO${}_{4}$)${}_{3}$ as a rare three-dimensional frustrated magnet hosting intertwined static and dynamic correlations.