  $^{75}$As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used,
together with M\"{o}ssbauer spectroscopy, to investigate the magnetic state
induced by Mn for Fe substitutions in F-doped LaFe$_{1-x}$Mn$_{x}$AsO
superconductors. The results show that $0.5$% of Mn doping is enough to
suppress the superconducting transition temperature $T_c$ from 27 K to zero and
to recover the magnetic structure observed in the parent undoped LaFeAsO. Also
the tetragonal to orthorhombic transition of the parent compound is recovered
by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR
spectra also show that a charge localization process is at play in the system.
Theoretical calculations using a realistic five-band model show that
correlation-enhanced RKKY exchange interactions between nearby Mn ions
stabilize the observed magnetic order, dominated by $Q_1=(\pi,0)$ and
$Q_2=(0,\pi)$ ordering vectors. These results give compelling evidence that
F-doped LaFeAsO is a strongly correlated electron system at the verge of an
electronic instability.
