AAP sources said the complainant, who requested anonymity, first approached a senior party leader with the grievance and then a "written complaint" was filed yesterday.
As Chief Minister Arvind Kejriwal and the party's top brass asked for more evidence to probe the matter, an audio clip of a purported conversation in which Khan was heard asking for a bribe from a builder was produced.
"When we heard the audio, we found that prima facie Khan was guilty and we decided to take action against him," said a party leader.
"We will refer his case to party Lokpal who will probe the matter," AAP spokesperson Deepak Bajpai said.
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Incidentally, Khan has become third AAP MLA and second minister to have his case probed by the party's internal ombudsman. The party Lokpal is already probing the fake degree case of former Delhi Law Minister Jitendra Singh Tomar and Kondli MLA Manoj Kumar.
A decision on Khan's fate will only be taken after the Lokpal's report, the spokesperson said.