Who introduced the Strong Atiyah Conjecture?

Recall that the conjecture says the following. Let $G$ be a group, $A$ a $n\times n$-matrix over ${\mathbb Z}G$. We view $A$ as a bounded operator $l^2(G)^n\to l^2(G)^n$. Let $K$ be the kernel of that operator and $p$ be the orthogonal projection onto $K$. Let $\delta$ be the function of $l^2(G)$ which is 1 on $1\in G$, and 0 everywere else. Let $e_i$ be the vector from $l^2(G)^n$ with $i$-th coordinate $\delta$, all other coordinates $0$, and $t_A$ be the sum of dot products $\sum_i \langle p(e_i), e_i\rangle$. The conjecture says that $t_A$ always belongs to $\frac1g\mathbb{Z}$ where $g$ is the least common multiple of the orders of finite subgroups of $G$ (so if $G$ is torsion-free, $t_A$ must be an integer).