LONGVIEW — A weekend power outage that affected 85,000 East Texas customers of AEP Southwestern Electric Power Co. was caused by an overload on part of the power grid after vegetation came into contact with two major power lines between Longview and Hallsville, the utility said Tuesday evening.

“We’re continuing to investigate the sequence of events to better understand what happened,” said Malcolm Smoak, SWEPCO president and CEO. “We are inspecting power lines and clearing additional trees and other vegetation in problem areas along transmission rights of way, where excessive rain this spring and summer has resulted in high vegetation growth.”

The Shreveport-based utility said initial reports from its investigation of the massive outage showed the first high-voltage power line came into contact with vegetation that had grown into it and caused a fault on the system about 4 p.m. Sunday. Heat and high customer load caused a second high-voltage power line to sag into vegetation growing below it, resulting in a second fault about 4:15 p.m.

SWEPCO said it began an emergency procedure of controlled outages to avoid further overloading the system. The total outages peaked at about 85,000 customers shortly after 7 p.m.

Power was restored to most affected customers by 11 p.m. Sunday. Work continued overnight to complete restoration of service to the remaining 500 customers, with a handful still without power by midday Monday, according to SWEPCO’s outage reporting system.

Most customers affected were in Longview, Marshall and the surrounding areas of Gregg and Harrison counties. But customers in a total of nine counties, including Marion, Rusk and Upshur, all suffered outages Sunday afternoon and evening.

The outage also was blamed for plant shutdowns at Eastman Chemical Co. south of Longview. Those shutdowns required the company to flare and burn a variety of gases and chemicals.

Eastman spokeswoman Kristin Parker said Monday the flares safely controlled process materials but might have emitted materials “in excess of the levels” Eastman is required to provide to the Texas Commission on Environmental Quality and the National Response Center.

TCEQ spokesman Andrew Keese Tuesday that Eastman submitted its initial air emissions event report.

“Do keep in mind that it’s the initial report from the company, which has two weeks after the end of the emissions event to submit its final report,” Keese said in a prepared statement. “Initial reports are often estimates.”

In the Texas portion of its market, SWEPCO has 185,000 customers. Including those in Northeast Texas, the Panhandle, Western Arkansas and Central Louisiana, it has 536,300 customers.