Journal of Lie Theory

\def\R{{\Bbb R}} \def\e{{\varepsilon}} \def\Id{\mathop{\rm Id}\nolimits} Let $L\subset V=\R^{k,l}$ be a maximally isotropic subspace. It is shown that any simply connected Lie group with a bi-invariant flat pseudo-Riemannian metric of signature $(k,l)$ is 2-step nilpotent and is defined by an element $\eta \in \Lambda^3L\subset \Lambda^3V$. If $\eta$ is of type $(3,0)+(0,3)$ with respect to a skew-symmetric endomorphism $J$ with $J^2=\e\Id$, then the Lie group ${\cal L}(\eta)$ is endowed with a left-invariant nearly K\"ahler structure if $\e =-1$ and with a left-invariant nearly para-K\"ahler structure if $\e =+1$. This construction exhausts all complete simply connected flat nearly (para-)K\"ahler manifolds. If $\eta \neq 0$ has rational coefficients with respect to some basis, then ${\cal L}(\eta)$ admits a lattice $\Gamma$, and the quotient $\Gamma\setminus {\cal L}(\eta)$ is a compact inhomogeneous nearly (para-)K\"ahler manifold. The first non-trivial example occurs in six dimensions.