Crews from the city of Lawrence and Kansas Department of Health and Environment traced the fumes to the ground beneath Priscilla's, 1206 W. 23rd St., said Mark Bradford, Lawrence-Douglas County Fire & Medical deputy chief.
Priscilla's is the site of a former gasoline station, Bradford said.
"There is either a tank underground that hasn't been removed and is leaking, or there is a pocket of gas that had leaked out and is now making its way into the sewer system," Bradford said.
The Kansas Department of Health and Environment was notified of the problem, and representatives were at the scene Monday evening searching for the tank, if one exists.
Friday, emergency crews were called to the area of 23rd and Alabama streets after inspectors from the Lawrence Utility Department detected higher-than-normal levels of a combustible gas. The equipment they carry doesn't specifically identify the gas, Bradford said.
Monday morning, utility workers detected higher-than-normal levels again, Bradford said. The source was thought to be a covered manhole near Priscilla's, he said.
No one was evacuated from the area either Friday or Monday.
Firefighters flushed the sewer system most of the day until the fumes had dissipated about 4 p.m., crews at the scene said.
Dan Wells, a KDHE environmental geologist, was at Priscilla's with firefighters late Monday afternoon.
Priscilla's leases the property. Douglas County tax records list the owner as Harold Shepard in care of Lawrence Gifts LLC.
Workers Monday night began digging a trench that they hoped would divert any further gas fumes away from the sewer system, Wells said. Today the digging will begin for the tank, he said.
-- Staff writer Mike Belt can be reached at 832-7165.