U.S. crude tumbled below $40 per barrel on Monday for the first time since April, as oil prices settled down nearly 4 percent on heightened worries of a crude glut despite peak summer fuel demand.

A nearly 15-percent slump in U.S. crude prices in July, the worst monthly loss in a year, also triggered liquidation as trading began for August.

Monday’s slide in crude prices also came after Marathon Petroleum unexpectedly shut its lone crude unit and an associated unit at its 212,000 barrel-per-day refinery in Robinson, Illinois, at the weekend, according to a source. The cause and duration of the outage were not clear.

“It’s stop-loss technical selling combined with sheer liquidation by those fearing we’ll soon be swimming in oil again,” said Phil Davis, trader at PSW Investments in San Diego, California. “We’ve had crude builds during the summer, when we were supposed to be having runaway draws from record driving.”

U.S. West Texas intermediate (WTI) crude plumbed $39.86, its lowest since April 20, before settling at $40.06, down $1.54, or 3.7 percent.

Brent crude closed down $1.39, or 3.2 percent, at $42.14 a barrel, after a session low of $41.87.