Prosecutors in the western cathedral city said that the 44-year-old attacker had specifically chosen Reker as his target so as to create "a climate of fear" among those volunteering to help refugees.
Reker was rushed to hospital and was in a coma during the remainder of the election, missing the fact that she took the city hall with 52.7 percent of the vote in the first round.
Four other people were also hurt during the attack.
The attack on Reker was "the current high point of a whole series of intimidations and threats" against those all over Germany who have chosen to help refugees, prosecutors said.
North Rhine-Westphalia security officials reported that the attacker had been linked to the far-right extremist scene in the state for years.
He was linked to the aggressive neo-Nazi "Free German Workers' Party" (FAP) in the 1990s until it was banned in 1995.