Chemical testing was carried out at a Leeds house today during a seven-hour emergency service operation.
It was wrongly believed that a potentially dangerous substance - possibly a kind of acid or acetone - may have been stored at a house on Frontline Close in Roundhay.
West Yorkshire Police confirmed that another force, Humberside Police, requested that officers attend but that the area was declared safe at around 8pm.
Police first attended at around 2pm and chemical, biological, radiological and nuclear (CBRN) officials were drafted in to assess the situation.
Fire service staff also turned out to conduct tests.
Witness Phoebe Miller said that around 20 emergency service vehicles were in the area during the operation, including ambulances and a hazard area response team.
Duty Inspector Carlton Young, of West Yorkshire Police, said: "We were requested to by another force to go and do a search of the address."
"There was some concern over chemicals. We have got to go through the right procedures," he said.
Insp Young added that what they thought could have been dangerous chemicals "was an unknown mixture, shown to be completely safe".
Activity was on going this evening but was winding down by about 9pm.