The ATS is probing his supply channel and chains in UP.
According to reports, the Punjab police had shared information about Ashish with its counterparts in Uttar Pradesh last week.
The ATS spokesman said that Ashish Singh was arrested from Tikri on Jadugar road in Civil Lines area of Haridwar.
He is an accused in a case of attempt to murder, rioting with deadly weapons, criminal conspiracy and harbouring offender in Punjab in 2018.
Ashish is considered to be the right hand of Gugni Grewal a.k.a Harmeet Singh chief of KLF and supplier of weapons to the force.
Ashish also has a role in murder of RSS and Shiv Sena worker.
After the murder of KLF chief on January 27, 2020, in Pakistan, the Punjab police traced one Sukhpreet Singh of Monga in Punjab and on his tip-off, Ashish was traced.
Additional Director General ATS, D.K. Thakur, said that Ashish was named in other cases including illegal sale of liquor. He came in touch with KLF chief while he was locked up in Patiala jail. Ashish had also supplied ten pistols of various bores to KLF chief.
"We are tracing the links of Ashish and his channel of procurement of weapons," said Thakur. He also said that the case was worked upon by the information of Punjab police while a hunt is on about other partners and associates of Ashish in Uttar Pradesh.
Sources said that call detail record of Ashish was scanned and revealed some numbers from Pakistan.
( With inputs from IANS )