Seck held the SS ranks of Unterscharführer and later Oberscharführer (staff sergeant). He was the commander of Jungfernhof concentration camp, near Riga, Latvia. His office was at the Gestapo headquarters in Riga on Reimerstrasse.
According to Joseph Berman, a Jewish man from Ventspils and a survivor of The Holocaust in Latvia, who was assigned to the work detail cleaning Seck's automobile, Seck was closely associated with Rudolf Lange, the main SS leader in occupied Latvia. Seck made it a habit to meet, at the Šķirotava Railway Station, trains of Jews deported from Germany, Austria, or Czechoslovakia. Theoretically these Jews were to be sent to the Riga Ghetto or the Jungfernhof or Salaspils concentration camps, but usually this did not occur, as Seck would instead take them to Biķernieki or Rumbula forests, near Riga, and shoot them.
Seck also traveled about Latvia, the Baltic states and Belarus with Nazi convoys to fight partisans or liquidate various camps and ghettos. The Gestapo maintained a clothing depot in Riga, on Peterholm Street, where the belongings of murdered Jews would be collected. Seck was seen at the clothing depot appropriating for himself suitcases of new clothing and jewelry. Seck personally beat and maltreated prisoners on a regular basis.
Seck was responsible for selecting between 1600 and 1700 Jews from among the Jungfernhof concentration camp inmates to be transported, on 26 March 1942, to the Biķernieki forest to be murdered in what became known as the Dünamünde Action.
Following the war, Seck was tried in West Germany before the Landsgericht Hamburg with other Nazi personnel who planned or participated in the murder of Jews in Latvia. In 1951, Seck was convicted and sentenced to life in prison. Among his crimes were his personal murder of eight Jews, including seven in Jungfernhof. The court described these murders as follows:
One day in January or February 1942, the witness Le., who was on the Hamburg transport that arrived in Jungfernhof in December 1941 where she worked in the sewing room, walked across the yard looking for Seck from whom she required further instructions regarding a sewing job for him.
She observed the accused leave the so-called men’s block driving five old men before him. The witness stopped near the witness; the five people had to stand to attention and then kneel (...) She addressed the accused with her question regarding the sewing work but received no answer. The accused repelled her with a movement of the hand and began to shoot the Jews one after the other with a shot in the back of the neck from a distance of 2 to 3 metres. When the witness tried to run away after the first shot, the accused threatened to shoot her if she did not remain. The witness was forced to witness the further shootings.
The accused then shot the other four Jews. Between the individual shootings he paused to insult his victims by calling them lazy or venereal dogs. (...)