To this mass graves evidence we can add the graves from the Bornhagen gas van killings that were excavated after the war, documented in this trial archived at JNSV Vol 7 Nr. 231b (LG Stuttgart 3 Ks 31/49, pp. 217-33), and cited by Browning & Matthaeus, p.542, n.144. Below I summarize the testimonies of perpetrators and other witnesses that were gathered by German courts in the 1960's. The main secondary sources used are Browning and Matthaeus (hereafter B/M) and Chapter 3 of Kershaw.
Greiser was executed by the Poles in 1946 but his trial records are also held in ZStL (see Kershaw, p.88n.). The highest level perpetrator to give testimony to the German authorities was the HSSPF for the Warthegau, Wilhelm Koppe. Significantly, Koppe had spent 15 years of freedom as a successful businessman before 1960 yet, like Eichmann and Stangl, he did not gather together any evidence during that time that would have supported a denier narrative of resettlement, despite the fact that he would clearly have known where the 'surviving' Jews went; when they went, and how. Greiser and Frank were similarly silent on any alternative fate for the Jews in their Polish and Nuremberg trial defences.
Such silence is a killer blow to denial. Instead Koppe, like the others, relied on a 'just following orders' defence. He claimed that, when Lange arrived in the Warthegau with his gas van, Greiser had confirmed to Koppe that Lange's mission was the result of a Fuehrer order (Kershaw, p.72). However, B/M (p.417) show that Koppe supplied personnel to Sk Lange through his IdS, Damzog, as well as recruiting a rotating squad of 80 Orpo for Chelmno from the BdO, Oskar Knofe. Furthermore, both Greiser and Koppe were promoted by Himmler on January 30, 1942, which suggests that the RFSS, HSSPF and Gauletier were in close harmony on the purpose of Chelmno.
Testimonies concerning the killing process are summarized by Roberto here and here and can be found on-line in the accounts of Malzmueller and Burmeister.
In addition, the design of the gas van itself, and its use of a gasoline engine, is detailed in the witness account of Polish factory worker Piaskowski:
However, at least three witnesses were able to see the vehicles from the short distance. Mr. Jozef Piaskowski (b. 1908) was employed in the Reichsstrassenbauamt in Kolo (former *Ostrowski factory). In the winter 1941/42 he was ordered to repair the damaged cooler in the biggest of Chelmno vans. Piaskowski was an experienced driver. He declared later that he has never seen the motor of this type. "The motor was a bit odd". "It was enormous". The most interesting in his report is the description of the exhaust system. He has noticed that the exhaust pipe was divided into three parts. First and third were done of metal as in normal cars. But, the central part was done of the elastic, "hydraulic" pipe which could joint both standard tubes or could be screwed to the holen the van's floor. After the repair of the cooler, when the motor was tested, so much exhaust fumes were produced that the air in the garage (size 30 m x 12 m) started immediately to be blue. The German bosses ordered to open all windows and doors. The workers who spent a very short time in the polluted air have got headache. The witness heard later their comments that the motor of this car uses 75 liters of petrol per 100 km, so twice more than normal motors do. Piaskowski stated that he had seen two military type gas-masks in the driver's cab. Piaskowski's colleague, Mr. Bronislaw Mankowski (b. 1882) confirmed his story and added that he had seen the van when the middle part of the exhaust tube was joint to the hole in the car's floor. Mankowski declared that he looked inside the box when the watchmen left their posts for a while. He had seen a hole covered with a perforated sheet iron in the middle of the wooden floor.As Roberto has noted, the testimony concerning the large consumption of petrol by the vehicle converges with Jeckeln's testimony in the USSR that gas vans had "too high gasoline consumption and cleaning problems."
Finally, due to the imperfect concealment of the killing site from a neighboring school, the early killings at Chelmno were witnessed by two ethnic German children, Nelli Lohrko and Else Semmler, who later testified to the West German authorities (B/M, p.542n.)